A bird that has to grumble (6)
I believe the answer is:
grouse
I believe this is a double definition.
'a bird' is the first definition.
(grouse is a kind of bird)
'to grumble' is the second definition.
(I've seen this before)
'that has' acts as a link.
(Other definitions for grouse that I've seen before include "Complain pettily" , "1D [GAMEBIRD] < having grumble" , "Gripe; game bird" , "Grumble like game-bird" , "Complain like gamebird" .)