Fit youngster has nothing for self-defence (6)
I believe the answer is:
aikido
'self-defence' is the definition.
(aikido is a kind of self-defence)
'fit youngster has nothing' is the wordplay.
'fit' becomes 'AI' (resembles Ak, old-fashioned slang for very good).
'youngster' becomes 'kid' (synonyms).
'has' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (looks like zero - 0).
'ai'+'kid'+'o'='AIKIDO'
'for' acts as a link.
(Other definitions for aikido that I've seen before include "Japanese martial art similar to judo" , "Martial art, akin to judo" , "Defensive moves" , "one of the martial arts" , "Japanese form of self-defence" .)