Fastening second snare (5)
I believe the answer is:
strap
'fastening' is the definition.
'second snare' is the wordplay.
'second' becomes 's' (common abbreviation).
'snare' becomes 'trap' (synonyms).
's'+'trap'='STRAP'
(Other definitions for strap that I've seen before include "Shoulder band" , "Fasten; punishment" , "Parts (anag.)" , "Secure" , "Belt; leather strip" .)