Resentful, having left omen about drink (6,5)
I believe the answer is:
bitter lemon
'drink' is the definition.
(I know that bitter lemon is a drink)
'resentful having left omen about' is the wordplay.
'resentful' becomes 'bitter' (synonyms).
'having' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'about' is an anagram indicator.
'omen' is an anagram of 'emon'.
'bitter'+'l'+'emon'='BITTER LEMON'
(Other definitions for bitter lemon that I've seen before include "Tart soft drink" , "Soft citrus drink" .)