Old woman’s about right, needing a cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'old woman's about right needing a' is the wordplay.
'old woman's' becomes 'ma's' (belongs to ma).
'about' becomes 'ca' (circa).
'right' becomes 'r' (common abbreviation).
'needing' means one lot of letters go next to another.
'mas'+'ca'+'r'+'a'='MASCARA'
(Other definitions for mascara that I've seen before include "Type of make-up" , "Eyelash darkener" , "Eyelash make-up" , "Make-up item" , "for application of lashes" .)