Fasteners for flies? (4)
I believe the answer is:
zips
I believe this is a double definition.
'fasteners' is the first definition.
(zip is a kind of fastener)
'flies?' is the second definition.
(zip can mean to move or fly around quickly)
'for' acts as a link.
(Other definitions for zips that I've seen before include "Shutters" , "Slide fasteners" , "Trouser fasteners" , "Dispatches" , "speeds" .)