To help, brought round a cold tart (4)
I believe the answer is:
acid
'tart' is the definition.
(both can mean bitter-tasting)
'to help brought round a cold' is the wordplay.
'to help' becomes 'aid' ('aid' can be a synonym of 'help').
'brought round' means one lot of letters goes inside another.
'a cold' becomes 'c' (eg on taps).
'aid' going around 'c' is 'ACID'.
(Other definitions for acid that I've seen before include "Sour tasting" , "Bitter substance" , "Sharp - tart - kind of house party" , "Sour to the taste" , "Opposite of alkali; sour" .)