Cosmetic mum used to conceal a blemish (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'mum used to conceal a blemish' is the wordplay.
'mum' becomes 'ma' (synonyms).
'used to conceal' means one lot of letters goes inside another.
'blemish' becomes 'scar' (scarring is a kind of blemishing).
'a'+'scar'='ascar'
'ma' placed around 'ascar' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "that accentuates lashes" , "Eyelash darkener" , "Maracas (anag.)" , "facial cosmetic" , "Make-up for eyelashes" .)