Mum covering a blemish in cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(I know that mascara is a type of make-up)
'mum covering a blemish' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'covering' means one lot of letters go next to another (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 "Makeup used to darken and thicken the eye lashes" , "Cosmetic for the eyes" , "that accentuates lashes" , "facial cosmetic" , "Eyelash darkener" .)