Lash thickener (7)
I believe the answer is:
mascara
'lash thickener' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mascara that I've seen before include "Eyelash darkener" , "Make-up for eyelashes" , "facial cosmetic" , "Eyelash make-up" , "Cosmetic for darkening the eyelashes" .)