Pets turn out to be vermin (4)
I believe the answer is:
pest
'vermin' is the definition.
'pets turn out' is the wordplay.
'turn out' indicates an anagram.
'pets' with letters rearranged gives 'PEST'.
'to be' is the link.
(Other definitions for pest that I've seen before include "Destructive insect or other animal" , "Troublesome man or animal" , "Menace" , "Troublesome person" , "Nuisance; half a European capital" .)