Fly on the fender (4)
I believe the answer is:
I believe this is a double definition.
'fly' is the first definition.
(I know that fly can be written as wing)
'the fender' is the second definition.
('wing' can be a synonym of 'fender')
'on' acts as a link.
(Other definitions for wing that I've seen before include "Arm adapted for flight" , "Slightly wound attacker" , "Footballer" , "Plane supporter" , "Chicken portion" .)