Small fastener for slow mover (5)
I believe the answer is:
snail
'slow mover' is the definition.
(snails move very slowly)
'small fastener' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fastener' becomes 'nail' (nail is a kind of fastener).
's'+'nail'='SNAIL'
'for' is the link.
(Other definitions for snail that I've seen before include "Not one of the fleet" , "dawdler" , "Edible terrestrial mollusc" , "Slow slithery animal" , "Slow-moving garden pest" .)