Could be pet's collar, missing top piece (6)
I believe the answer is:
rabbit
'could be pet's' is the definition.
'collar missing top piece' is the wordplay.
'collar' becomes 'grab'.
'missing top' means to remove the first letter.
'piece' becomes 'bit' (bit is a kind of piece).
'grab' with its first letter taken away is 'rab'.
'rab'+'bit'='RABBIT'
(Other definitions for rabbit that I've seen before include "Meat" , "Animal called coney" , "Weak player" , "Bunny; chatter idly" , "Fast-breeding hopper" .)