Standard box, not small (3)
I believe the answer is:
par
'standard' is the definition.
(thesaurus)
'box not small' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'not' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'spar' with 's' taken out is 'PAR'.
(Other definitions for par that I've seen before include "Black music" , "Average score for round of golf" , "Regular score" , "Expected number of strokes for hole (golf)" , "Rating of strokes required for hole (golf)" .)