Favourite training tons (3)
I believe the answer is:
pet
'favourite' is the definition.
('pet' can be a synonym of 'favourite')
'training tons' is the wordplay.
'training' becomes 'pe'.
'tons' becomes 't'.
'pe'+'t'='PET'
(Other definitions for pet that I've seen before include "blue-eyed boy?" , "The darling one in a small temper" , "Cherished favourite" , "Caress - budgie, perhaps?" , "Favourite or house animal" .)