Mother covering a blemish in cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'mother covering a blemish' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'covering' says to put 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 "that accentuates lashes" , "Make-up for eyelashes" , "Eyelash darkener" , "Type of make-up" , "for application of lashes" .)