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' means one lot of letters goes inside another.
'one key' becomes 'E' (a musical key).
'pt' going around '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" .)