Point out one key as favourite (3)
I believe the answer is:
pet
'favourite' is the definition.
(synonyms)
'point out one key' is the wordplay.
'point' becomes 'pt' (abbreviation for point).
'out' indicates putting letters inside.
'one key' becomes 'E' (musical key).
'pt' enclosing 'e' is 'PET'.
'as' acts as a 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" .)