Fastens neckwear (4)
I believe the answer is:
ties
This clue is a double definition.
'fastens' is the first definition.
(tying is a kind of fastening)
'neckwear' is the second definition.
(I've seen this in another clue)
(Other definitions for ties that I've seen before include "Binds with male neckwear" , "Links" , "Fastens with a knot" , "Items of neckwear" , "allegiances" .)