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