Clash-likes are one of those genres that has a lot of room for tweaks, changes or totally new stuff; but you still know one when you see one. Here’s my list of what usually makes one:

• Solo “opposite tower defense” gameplay. You attack other players, but you’re not playing head-to-head. You only fight their AI-controlled tower defense — not their army. Likewise, when someone attacks you, your offensive army does nothing. They only fight your tower defense.
• Persistent progress. Once you get something, you can never lose it. Being attacked steals some gold you were saving for the next upgrade, but otherwise your defenses spring fully back after each attack. You can stop playing for a week and not be set back. Your offensive army isn’t even harmed when your village is destroyed.
• Very dumb, obvious AI. This is considered a feature. Defenses will target the nearest enemy, ignoring a deadly but soft wizard for a slightly closer knight. Likewise your attackers can’t be controlled after placing them — they attack the nearest buildings. If you can trick the AI into walking around your base while being blasted, good for you.
• Short, time-limited, pauseless battles. You can spend a while planning your army, figuring out a strategy, trying to figure out what went wrong … . But the actual attack is 3 minutes at most. Depending on your army and strategy, you may even lose by running out of time. And after the attack, that’s it; you’re done with that player.
• A medium amount of tap-based decisions during an attack. Generally targetable spells and troop power-ups. Sometimes limited control over a leader troop. You’re not tapping as fast as you can, but you have to concentrate and will forgot things until you make a plan and get practice.
• The attacks are real games. There’s a skill in picking an army, where and when to place troops, and when to cast spells. A good player can do much better than a beginner.
• Troop selection is a real choice. You never get a new troop which replaces an old one. In fact, when a new troop is unlocked, you’re supposed to wonder how in the heck you’re going to use it.
• You can’t pick on a particular person, and they can’t pick on you. Fights are against randomly chosen opponents. The same goes for guilds. A guild war is a 1-time event, against a randomly chosen guild, where the attacks don’t even steal gold.
• Likewise, other players can’t directly help you. There’s usually a small friendship mechanic. Altogether, it’s a mostly solo game. You don’t need a big guild to do well. This is in contrast to things like Game of War, where you’re nothing without a big active guild.
• Gameplay is attacking to gather gold, paying for all the allowed upgrades, then waiting. That may take a while, or may be quick. But starting all allowed upgrades is the goal and the natural stopping time.
• This is just aesthetics, but it’s a recognizable feature: everything you can do comes from a building in your village map, where it also becomes part of your tower defense. Your troop upgrade buildings, spell creation buildings … don’t fight back, but they’re there, next to your guns, soaking up damage.
• Also just aesthetics: each building and troop gets a little different look as it’s upgraded. When you see someone’s base, or an attack replay, you can mostly figure out everything’s level from the pictures.
• Planning your defense is also a real game. There are lots of options, no optimal strategy, and a thoughtful defense can matter (but, since you aren’t playing during an attack, you can just copy a base design from the internet.)
• You have to get better to progress. At various points, people will be stealing gold as fast as you can steal it from others. You’ll have to either get better at attacking, build a better defense, or a combination of both.  In other words, it’s a game – not a fancy Cow Clicker.
• Very, very simple resource generation. Tap to collect, and nothing else. They could easily make it more complex – like a tiny farmville. But the game is about fighting. The resource generators are just a reason for all the attacking.
• Freemium, but you can’t buy anything you couldn’t get just by playing a lot. No one will crush your base because they bought all of the weekly power-ups. In technical terms, it’s Pay-to-Progress, not Pay-to-Win.