Lash lengthener (7)
I believe the answer is:
mascara
'lash lengthener' 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 "Makeup used to darken and thicken the eye lashes" , "that accentuates lashes" , "facial cosmetic" , "Cosmetic for darkening the eyelashes" , "Eyelash cosmetic" .)