Cold tart (6)
I believe the answer is:
bitter
This clue is a double definition.
'cold' is the first definition.
(similar in meaning)
'tart' is the second definition.
(thesaurus)
(Other definitions for bitter that I've seen before include "Beer; not sweet" , "Sour tasting, like English ale" , "Resentful or angry through disappointment" , "Sharp-tasting or resentful" , "Tart or resentful" .)