Complain to bird (6)
This clue is a double definition.
'complain' is the first definition.
(grouse can mean to complain or grumble)
'bird' is the second definition.
(I know that grouse is a type of wildfowl)
'to' is the link.
(Other definitions for grouse that I've seen before include "Petty complaint" , "No shooting it before 12 August" , "Grumble like game-bird" , "Bellyache" , "1D [GAMEBIRD] < having grumble" .)
I believe this is a double definition.
'complain' is the first definition.
(I know that rail is a more specific form of complain)
'bird' is the second definition.
(a wading bird)
'to' acts as a link.
(Other definitions for rail that I've seen before include "The permanent way" , "Form of transport" , "Horizontal bar - bird - complain" , "Bar for hanging; bird as corncrake" , "Safety bar; bird" .)