Feels bitter about finding gifts unopened (7)
I believe the answer is:
resents
'feels bitter about' is the definition.
('resent' can be a synonym of 'feel bitter about')
'gifts unopened' is the wordplay.
'gifts' becomes 'presents' (present is a kind of gift).
'unopened' means to remove the first letter (without opening letter).
'presents' with its first letter taken off is 'RESENTS'.
'finding' acts as a link.
(Other definitions for resents that I've seen before include "Begrudges" , "Takes offence" , "grudges" .)