A box with insipid artificial sweetener (9)
I believe the answer is:
aspartame
'artificial sweetener' is the definition.
(I've seen this before)
'a box with insipid' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'with' means one lot of letters go next to another.
'insipid' becomes 'tame'.
'a'+'spar'+'tame'='ASPARTAME'
(Other definitions for aspartame that I've seen before include "Strong artificial sweetener -- same apart (anag)" , "What sweetens" , "Sugar substitute" .)