Favourite small creatures (4)
I believe the answer is:
pets
'creatures' is the definition.
(pet is a kind of creature)
'favourite small' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'small' becomes 's' (abbreviation - e.g. clothes size).
'pet'+'s'='PETS'
(Other definitions for pets that I've seen before include "Domesticated animals" , "Animals at home" , "Domestic animals" , "Little darlings - step on them" .)