A box with domestic sweetener (9)
I believe the answer is:
aspartame
'sweetener' is the definition.
(aspartame is a kind of sweetener)
'a box with domestic' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'with' says to put letters next to each other.
'domestic' becomes 'tame' ('tame' can be similar in meaning to 'domestic').
'a'+'spar'+'tame'='ASPARTAME'
(Other definitions for aspartame that I've seen before include "What sweetens" , "Sugar substitute" , "Strong artificial sweetener -- same apart (anag)" .)