Cat perhaps in bad mood (3)
I believe the answer is:
pet
I believe this clue is a double definition.
'cat perhaps' is the first definition.
(cat is a type of pet)
'bad mood' is the second definition.
(informal term for sulking)
'in' acts as a link.
(Other definitions for pet that I've seen before include "Favourite, perhaps a domestic animal" , "Particular favourite" , "Preferred" , "Domesticated animal" , "Animal as a companion" .)