A flyer's fleece (4)
I believe the answer is:
rook
This clue is a double definition.
'a flyer's' is the first definition.
(rook is a type of bird)
'fleece' is the second definition.
(thesaurus)
(Other definitions for rook that I've seen before include "man in marginalised position, initially" , "Chess piece also called castle" , "Chesspiece otherwise known as a castle" , "Bird; playing piece" , "Fleece - bird" .)