Old woman covering a blemish in make-up (7)
I believe the answer is:
mascara
'make-up' is the definition.
(I've seen this before)
'old woman covering a blemish' is the wordplay.
'old woman' becomes 'ma' (I've seen this in other clues).
'covering' is a charade indicator (letters next to each other) (on top of in down clue).
'blemish' becomes 'scar' (scarring is a kind of blemishing**).
'a' put after 'scar' is 'scara'.
'ma'+'scara'='MASCARA'
'in' acts as a link.
(Other definitions for mascara that I've seen before include "Eyelash cosmetic" , "Eyelash make-up" , "Cosmetic for darkening the eyelashes" , "Type of make-up" , "Maracas (anag.)" .)