Loosens crewel segment to get to the fastener (5)
I believe the answer is:
screw
'the fastener' is the definition.
(screw is a kind of fastener)
'loosens crewel segment to get' is the wordplay.
'to get' indicates a hidden word (I've seen 'gets' mean this).
'SCREW' is hidden within 'loosens crewel segment'.
'to' acts as a link.
(Other definitions for screw that I've seen before include "Bring pressure to bear" , "Threaded pin" , "Wood fastener" , "Fastener; propeller" , "Prison officer - salary" .)