Mum covering a mark needs cosmetic (7)
I believe the answer is:
mascara
'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'.
'ma'+'scara'='MASCARA'
'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" .)