A Japanese martial art (6)
'a japanese martial art' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aikido that I've seen before include "a particular form of art" , "Martial art, akin to judo" , "Japanese form of self-defence" , "one of the martial arts" , "Japanese martial art similar to judo" .)
(Other definitions for jiujitsu that I've seen before include "Japanese system of unarmed combat" , "Self-defence method without weapons" , "Martial art" .)