First class youngster has nothing but a martial art (6)
I believe the answer is:
aikido
'a martial art' is the definition.
(aikido is a kind of martial art)
'first class youngster has nothing' is the wordplay.
'first class' becomes 'ai' (Ak - top quality).
'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'
'but' is the link.
(Other definitions for aikido that I've seen before include "one of the martial arts" , "a particular form of art" , "Martial art, akin to judo" , "Japanese form of self-defence" , "Defensive moves" .)