One's caught by zip fastener (4)
I believe the answer is:
nail
'fastener' is the definition.
(nail is a kind of fastener)
'one's caught by zip' is the wordplay.
'one' becomes 'a'.
'caught by' is an insertion indicator.
'zip' becomes 'nil' ('nil' can be a synonym of 'zip'**).
'a' placed inside 'nil' is 'NAIL'.
(Other definitions for nail that I've seen before include "Spike, tack" , "Fix with a metal spike" , "for the carpenter?" , "Fastening device" , "thing for keeping things together" .)