Cosmetic, best applied under cheek (4,5)
I believe the answer is:
face cream
'cosmetic' is the definition.
(type of cosmetic)
'best applied under cheek' is the wordplay.
'best' becomes 'cream' (as in 'the cream of the crop').
'applied under' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'cheek' becomes 'face' (synonyms).
'cream' after 'face' is 'FACE-CREAM'.
(Other definitions for face cream that I've seen before include "Mug gets this application" , "Mecca fare (anag.)" , "Skin cosmetic" , "Cosmetic designed to improve complexion" .)