Fasten the neckwear (3)
I believe the answer is:
tie
'fasten the neckwear' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tie that I've seen before include "Bind; be equal" , "Lace" , "Bind with a cord" , "Knot; link" , "Accessory" .)