More sentries have to feel bitter (6)
I believe the answer is:
resent
'feel bitter' is the definition.
(I've seen this before)
'more sentries have' is the wordplay.
'have' indicates the answer is hidden within the clue.
'RESENT' can be found hidden inside 'more sentries'.
'to' acts as a link.
(Other definitions for resent that I've seen before include "Take exception to" , "Find objectionable" , "Be offended by" , "Feel hurt, insulted, about" , "Dislike" .)