A mark covered by mother's cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'a mark covered by mother's' is the wordplay.
'mark' becomes 'scar' ('scar' can be a synonym of 'mark').
'covered by' indicates putting letters inside.
'mother' becomes 'ma' (ma is a kind of mother).
'a'+'scar'='ascar'
'ascar' inserted within 'ma' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "Eyelash cosmetic" , "Make-up for eyelashes" , "that accentuates lashes" , "Cosmetic for darkening the eyelashes" , "facial cosmetic" .)