Fastener in cask pulled up (3)
I believe the answer is:
nut
'fastener' is the definition.
(I have seen 'Mechanical fastener' mean 'nut' so perhaps 'fastener' could also mean 'nut')
'cask pulled up' is the wordplay.
'cask' becomes 'tun' (tun is a kind of cask).
'pulled up' says the letters should be written backwards.
'tun' in reverse letter order is 'NUT'.
'in' is the link.
(Other definitions for nut that I've seen before include "Enthusiast (slang)" , "Hard dry fruit - enthusiast" , "Hard fruit; obsessive" , "Cashew or hazel" , "Enthusiastic person" .)