A blemish concealed by old woman's cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'a blemish concealed by old woman's' is the wordplay.
'blemish' becomes 'scar' (scarring is a kind of blemishing**).
'concealed by' means one lot of letters goes inside another.
'old woman' becomes 'ma' (I've seen this in other clues).
'a'+'scar'='ascar'
'ascar' going into 'ma' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "Make-up for eyelashes" , "could be found in handbag, perhaps" , "Cosmetic for darkening the eyelashes" , "Eyelash darkener" , "Make-up item" .)