Nothing holds a fastener (4)
I believe the answer is:
nail
'fastener' is the definition.
(I know that nail is a type of fastener)
'nothing holds a' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'holds' is an insertion indicator.
'nil' enclosing 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Hit this on the head to be bang-on" , "Fixer (on finger?)" , "Catch - fastener" , "Arrest (informal) - fix" , "Brad, for example" .)