Dog favourite, cat superior (7)
I believe the answer is:
'dog' is the definition.
(whippet is a kind of dog)
'favourite cat superior' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'cat' becomes 'whip' (cat o' nine tails or cat is a type of whip).
'superior' says to put letters next to each other.
'pet' put after 'whip' is 'WHIPPET'.
(Other definitions for whippet that I've seen before include "Small greyhound-type dog" , "Dog bred for racing" , "Dog similar to a greyhound" , "Racing-dog" , "dog class?" .)