Short clues like that usually hinge on letter count and crossing letters, so I treat this like a little logic puzzle. If the grid wants a four-letter fill, my brain immediately jumps to judo or sumo. Judo is extremely common in crosswords because it’s short, internationally recognized, and fits cleanly; sumo also pops up when the clue leans toward traditional Japanese wrestling rather than the more modern martial arts.
If the pattern allows more letters, I scan for karate, aikido, kendo, or one of the spellings of jujutsu/jujitsu. Crosswords sometimes prefer the simpler romanizations without hyphens, and sometimes the grid theme nudges you toward a specific spelling. So I usually pencil in judo first, then test crossing letters; if they force a different vowel pattern I switch to kendo or aikido. I love how a few crossings can lock in the right martial art and make the whole section click—it's oddly satisfying.