Complain to a flyer (6)
I believe the answer is:
grouse
This clue is a double definition.
'complain' is the first definition.
(I know that grouse is a more specific form of the action complain)
'to a flyer' is the second definition.
(something which flies)
(Other definitions for grouse that I've seen before include "Complain pettily" , "Plump game bird" , "Petty complaint" , "excellent in New Zealand" , "See game bird" .)