Old woman’s vehicle associated with a cosmetic (7)
I believe the answer is:
mascara
'cosmetic' is the definition.
(mascara is a kind of cosmetic)
'old woman's vehicle associated with a' is the wordplay.
'old woman's' becomes 'mas' (belongs to ma).
'vehicle' becomes 'car' (car is a kind of vehicle).
'associated with' says to put letters next to each other.
'mas'+'car'+'a'='MASCARA'
(Other definitions for mascara that I've seen before include "Make-up for eyelashes" , "Eyelash darkener" , "for application of lashes" , "Makeup used to darken and thicken the eye lashes" , "Cosmetic for the eyes" .)