A disfigurement concealed by Mum's cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'a disfigurement concealed by mum's' is the wordplay.
'disfigurement' becomes 'scar' (I've seen this before).
'concealed by' indicates putting letters inside.
'mum' becomes 'ma' (synonyms).
'a'+'scar'='ascar'
'ascar' placed within 'ma' is 'MASCARA'.
(Other definitions for mascara that I've seen before include "facial cosmetic" , "Eyelash darkener" , "Eyelash cosmetic" , "for application of lashes" , "Makeup used to darken and thicken the eye lashes" .)