Sailor not entirely bitter (3)
I believe the answer is:
tar
'sailor' is the definition.
(tar can mean a sailor)
'not entirely bitter' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'bitter' becomes 'tart' (tart can mean bitter-tasting).
'tart' with its last letter taken off is 'TAR'.
(Other definitions for tar that I've seen before include "Sailor (colloq.)" , "Dark viscid substance" , "prepare for feathers?" , "'Thick, dark, sticky liquid (3)'" , "one may need to pay on boat" .)