Holds and locks in this fit youngster with nothing (6)
I believe the answer is:
aikido
'holds and locks in this' is the definition.
(form of self-defence using holds and locks)
'fit youngster with nothing' is the wordplay.
'fit' becomes 'AI' (resembles Ak, old-fashioned slang for very good).
'youngster' becomes 'kid' (synonyms).
'with' says to put letters next to each other.
'nothing' becomes 'O' (O resembles 0).
'ai'+'kid'+'o'='AIKIDO'
(Other definitions for aikido that I've seen before include "a particular form of art" , "Japanese martial art similar to judo" , "Defensive moves" , "Martial art, akin to judo" , "one of the martial arts" .)