A blemish covered by mother's cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'a blemish covered by mother's' is the wordplay.
'blemish' becomes 'scar' (scarring is a kind of blemishing**).
'covered by' means one lot of letters goes inside another.
'mother' becomes 'ma' (ma is a kind of mother).
'a'+'scar'='ascar'
'ascar' going within 'ma' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "Eyelash cosmetic" , "black application" , "Make-up item" , "Cosmetic for darkening the eyelashes" , "Maracas (anag.)" .)