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