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