Favourite dog (3)
I believe the answer is:
pet
This clue is a double definition.
'favourite' is the first definition.
(synonyms)
'dog' is the second definition.
(dogs are often kept as pets)
(Other definitions for pet that I've seen before include "Tamed animal; sulk" , "Favourite, darling" , "Bad humour; darling" , "The darling one in a small temper" , "Particular favourite" .)