Complains bitterly (5)
I believe the answer is:
rails
'complains bitterly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rails that I've seen before include "secretive birds" , "Makes a protest" , "Wading birds - dysfunctional person has come off them" , "Tracks or barriers" , "Train tracks" .)