Favourite gerbil, say (3)
I believe the answer is:
pet
I believe this is a double definition.
'favourite' is the first definition.
(synonyms)
'gerbil say' is the second definition.
(gerbils are often kept as pets)
(Other definitions for pet that I've seen before include "Favourite; fit of the sulks" , "Bad humour; darling" , "Particular favourite" , "Favourite pupil" , "Preferred" .)