Mum covering a mark needs cosmetic (7)
I believe the answer is:
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'mum covering a mark' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'covering' is a charade indicator (letters next to each other) (on top of in down clue).
'mark' becomes 'scar' (synonyms).
'a' after 'scar' is 'scara'.
'needs' is the link.
(Other definitions for mascara that I've seen before include "black application" , "could be found in handbag, perhaps" , "Eyelash darkener" , "Type of make-up" , "Cosmetic for the eyes" .)