Complain bitterly (4)
I believe the answer is:
rail
'complain bitterly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rail that I've seen before include "Track - complain - bird" , "Supported bar - inveigh" , "Scold, berate" , "Bar - marsh bird" , "Water bird; bar" .)