More sentries get to feel bitter (6)
I believe the answer is:
resent
'feel bitter' is the definition.
(I've seen this before)
'more sentries get' is the wordplay.
'get' indicates a hidden word (I've seen 'gets' mean this).
'RESENT' is hidden within 'more sentries'.
'to' acts as a link.
(Other definitions for resent that I've seen before include "Feel bad about" , "Dispatched again - take amiss" , "Object to" , "given another posting?" , "Feel or show annoyance" .)