Cosmetic for small mark under cheek (8)
I believe the answer is:
lipstick
'cosmetic' is the definition.
(lipstick is a kind of cosmetic)
'small mark under cheek' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mark' becomes 'tick' (I've seen this before).
'under' says to put letters next to each other.
'cheek' becomes 'lip' (lip is a kind of cheek).
's'+'tick'='stick'
'stick' after 'lip' is 'LIPSTICK'.
'for' is the link.
(Other definitions for lipstick that I've seen before include "Cosmetic used to colour around the mouth" , "Skit clip about a cosmetic" , "Type of paint" , "Cosmetic item" , "Coloured cosmetic" .)