Pets turn out to be vermin (4)
I believe the answer is:
pest
'to be vermin' is the definition.
'pets turn' is the wordplay.
'turn' indicates anagramming the letters.
'pets' with letters rearranged gives 'PEST'.
'out' acts as a 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" .)