Prats ruined fastening (5)
I believe the answer is:
strap
'fastening' is the definition.
'prats ruined' is the wordplay.
'ruined' is an anagram indicator.
'prats' is an anagram of 'STRAP'.
(Other definitions for strap that I've seen before include "Length or loop, of leather perhaps" , "Parts of a long piece of leather" , "Fastening strip" , "Narrow strip of leather" , "Shoulder band" .)