Resentful at fruit in beverage (6,5)
I believe the answer is:
bitter lemon
'beverage' is the definition.
(bitter lemon is a kind of beverage)
'resentful at fruit' is the wordplay.
'resentful' becomes 'bitter' (synonyms).
'at' means one lot of letters go next to another.
'fruit' becomes 'lemon' (lemon is a kind of fruit).
'bitter'+'lemon'='BITTER LEMON'
'in' acts as a link.
(Other definitions for bitter lemon that I've seen before include "Soft citrus drink" , "Tart soft drink" .)