Best child has ordinary means of defence (6)
I believe the answer is:
aikido
'means of defence' is the definition.
(martial art)
'best child has ordinary' is the wordplay.
'best' becomes 'ai'.
'child' becomes 'kid' ('kid' can be a synonym of 'child'**).
'has' is a charade indicator (letters next to each other).
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'ai'+'kid'+'o'='AIKIDO'
(Other definitions for aikido that I've seen before include "a particular form of art" , "one of the martial arts" , "Japanese form of self-defence" , "Japanese martial art similar to judo" , "Martial art, akin to judo" .)