Pets turned out to be vermin (4)
I believe the answer is:
pest
'vermin' is the definition.
'pets turned out' is the wordplay.
'turned out' indicates an anagram (turn out can mean to organise).
'pets' is an anagram of 'PEST'.
'to be' acts as a link.
(Other definitions for pest that I've seen before include "Destructive insect or other animal" , "Troublesome person" , "Plague" , "Destructive creature" , "A nuisance - step on it" .)