All right. It doesn't need defining since it's apparent, so how difficult and how effective is the 'Thunderclap Strike' that I want my warrior to perform? Since it doesn't need defining in the books because of a lifetime of living in the real world you should have little problem with doing that. And no, it's not anime or wuxia; it appears in medieval German fighting manuals.
Okay, 5-seconds on Google shows me that "thunderclap strike" is the common move where you cup your hands and strike your enemy on one or both ears. I imagine if any player was trying to do that, they'd be able to explain as much.
Seems simple. I imagine anyone trained in martial arts or unarmed fighting knows the move.
It's described as basic self-defense. So it really shouldn't be an advanced move you need a particular class feature to use. It'd be silly to say "You physically cannot slap someone on the ear unless you have the Cupping Ear Slap maneuver".
I'd treat it as an unarmed strike that does no damage but forces the target to make a saving throw or be stunned for a round. Until the end of their turn. If that seems to strong in play I'd change it to unable to take reactions and disadvantage on attacks and Perception checks.
Bam. 30-seconds of DMing.
-edit-
This is actually a pretty good example of rulings over rules.
You can't possibly have rules for every martial arts move ever. Otherwise we need rules for a punch to the solar plexus, knee to the groin, fist in the throat, kick to the knee, elbow blow to the face, heel of hand to the nose, headbutt, headlock, sleeper hold, etc. There's a near infinite amount of moves.
Limiting them all to feats or special tactics either means having a giant list of moves anyone can do but has to be referenced (and means there are good moves and inoptimal trap moves) or having a giant list you can only attempt with training.
Plus there are the niche cases. I ruled above that I wouldn't allow damage. It was strictly stun or damage (like a push w/o a maneuver).
But what about the monk? If anyone should be good at that move, it's the monk. And if I was just making a hard, written rule I would have omitted the monk. Ostensibly, "thunderclap strike" is just stunning strike and already in the game and they're just flavouring it (really, cupping the hand could just be a way to flavour an attack and just deal damage, and the stunning is just my changing the effect). But, arguably, you could have the monk just deal fist damage but not ability damage and stun (or vise versa) and still stun. But even if a book did include a monk exception, what happens if in six months the game includes a "brawler" fighter or a "knuckleduster" barbarian? Then you need an exception to the exception. Or the rule has to be that little bit more complex or carefully worded to include other classes.
But since it's a ruling not a rule, I can just change by ruling if "stunned" is too good, if new content is added, or if I forgot to account for existing content.
I don't need all new rules to explain "thunderclap strike" since the rules for damage, unarmed attacks, stunning, disadvantage, and the like are already in the game. And there's the push example that shows you can trade a damaging attack for a status effect. And I don't need the rules of the game to explain to me that "thunderclap strike" is physically possible since I've seen it in dozens of martial arts movies or read of the move being used in books. It's physically possible, so a character can try it.
Unlike magic, where unless there's a spell called "
reverse aging" you don't know if a wizard or cleric can make themselves young, despite that being a common trope also seen in movies and books. The rules for restoring ones youth and limits on the magic need to be explained and established, because it's very much not something everyone can do.