Food additive in a box ruined meat (9)
I believe the answer is:
aspartame
'food additive' is the definition.
'a box ruined meat' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'ruined' is an anagram indicator.
'meat' is an anagram of 'tame'.
'a'+'spar'+'tame'='ASPARTAME'
'in' is the link.
(Other definitions for aspartame that I've seen before include "What sweetens" , "Strong artificial sweetener -- same apart (anag)" , "Sugar substitute" .)