Composition of mascara? (4-2)
I believe the answer is:
make-up
This clue is a double definition.
'composition' is the first definition.
(something's make-up is its composition)
'mascara?' is the second definition.
(mascara is a type of make-up)
'of' is the link.
(Other definitions for make-up that I've seen before include "Compose - cosmetics" , "Cosmetics for self-beautification" , "Lipstick, mascara etc" , "Heal breach, become friends again" , "Constitution" .)