Fuss about one child not finishing martial art (6)
I believe the answer is:
'martial art' is the definition.
(aikido is a kind of martial art)
'fuss about one child not finishing' is the wordplay.
'fuss' becomes 'ado' (synonyms).
'about' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'child' becomes 'kid' (synonyms).
'not finishing' means to remove the last letter.
'ikid' with its final letter taken away is 'iki'.
'ado' going around 'iki' is 'AIKIDO'.
(Other definitions for aikido that I've seen before include "Defensive moves", "one of the martial arts", "a particular form of art", "Japanese martial art similar to judo", "Japanese form of self-defence".)