Help to wrap cold tart (4)
I believe the answer is:
acid
'tart' is the definition.
(both can mean bitter-tasting)
'help to wrap cold' is the wordplay.
'help' becomes 'aid' (synonyms).
'to wrap' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'aid' placed around 'c' is 'ACID'.
(Other definitions for acid that I've seen before include "Caustic liquid of many kinds" , "- - -drop, sweet" , "bad rain" , "Acerbic" , "Chemical compound" .)