Adult 1 — child 0 in martial art (6)
I believe the answer is:
aikido
'martial art' is the definition.
(aikido is a kind of martial art)
'adult 1 child 0' is the wordplay.
'adult' becomes 'a' (abbreviation).
'k' becomes 'I' (roman numeral).
'child' becomes 'kid' (synonyms).
'0' becomes 'o' (I've seen this in other clues).
'a'+'i'+'kid'+'o'='AIKIDO'
'in' is the link.
(Other definitions for aikido that I've seen before include "a particular form of art" , "Martial art, akin to judo" , "one of the martial arts" , "Defensive moves" , "Japanese form of self-defence" .)