Feel bitter about unopened gift (6)
I believe the answer is:
resent
'feel bitter' is the definition.
(I've seen this before)
'unopened gift' is the wordplay.
'unopened' means to remove the first letter.
'gift' becomes 'present' (present is a kind of gift).
'present' with its initial letter taken off is 'RESENT'.
'about' is the link.
(Other definitions for resent that I've seen before include "grudge" , "Feel affronted by" , "Take offence at" , "Dislike" , "Consider (something) to be an affront" .)