Old woman covering a blemish in cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'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' after 'scar' is 'scara'.
'ma'+'scara'='MASCARA'
'in' is the link.
(Other definitions for mascara that I've seen before include "facial cosmetic" , "that accentuates lashes" , "Make-up for eyelashes" , "Make-up item" , "Eyelash cosmetic" .)