Point out one key as favourite (3)
I believe the answer is:
pet
'favourite' is the definition.
('pet' can be a synonym of 'favourite')
'point out one key' is the wordplay.
'point' becomes 'pt' (**).
'out' is an insertion indicator.
'one key' becomes 'e' (I've seen this in other clues).
'pt' enclosing 'e' is 'PET'.
'as' is the link.
(Other definitions for pet that I've seen before include "Caress or fondle, as with favourite" , "Tame animal kept for companionship" , "Domesticated animal" , "Favourite, may be teacher's" , "Favourite; fit of the sulks" .)