Not all ripe tomatoes are “sweet” (3)
I believe the answer is:
pet
'are sweet' is the definition.
(thesaurus)
'not all ripe tomatoes' is the wordplay.
'not all' indicates the answer is hidden within the clue.
'PET' is hidden in the letters of 'ripe tomatoes'.
(Other definitions for pet that I've seen before include "Favourite, darling" , "Paddy" , "Favourite, perhaps a domestic animal" , "Animal as a companion" , "Domesticated animal" .)