Applied to cheek, the best cosmetic (4,5)
I believe the answer is:
face cream
'cosmetic' is the definition.
(type of cosmetic)
'applied to cheek the best' is the wordplay.
'applied to' means one lot of letters go next to another.
'cheek' becomes 'face' (synonyms).
'the best' becomes 'cream' (as in 'the cream of the crop').
'face' next to 'cream' is 'FACE-CREAM'.
(Other definitions for face cream that I've seen before include "Mecca fare (anag.)" , "Mug gets this application" , "Cosmetic designed to improve complexion" , "Skin cosmetic" .)