Complains bitterly as Republican is sick (5)
I believe the answer is:
rails
'complains bitterly' is the definition.
'rails' can be an answer for 'complains' (railing is a kind of complaining). I also know that 'bitterly' relates to this answer.
'republican is sick' is the wordplay.
'republican' becomes 'r' (abbreviation).
'is sick' becomes 'ails' ('ail' can be a synonym of 'be sick').
'r'+'ails'='RAILS'
'as' acts as a link.
(Other definitions for rails that I've seen before include "secretive birds" , "For trains" , "Wading birds - dysfunctional person has come off them" , "Metal bars of a barrier" , "Tracks or barriers" .)