Make-up mother's used to conceal a blemish (7)
I believe the answer is:
mascara
'make-up' is the definition.
(I've seen this before)
'mother's used to conceal a blemish' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'used to conceal' is an insertion indicator.
'blemish' becomes 'scar' (scarring is a kind of blemishing).
'a'+'scar'='ascar'
'ma' going around 'ascar' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "Eyelash darkener" , "black application" , "Type of make-up" , "for application of lashes" , "Eyelash cosmetic" .)