I give black marks for going on the lash (7)
I believe the answer is:
mascara
'lash' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for mascara that I've seen before include "Make-up item" , "Eyelash darkener" , "Eyelash make-up" , "that accentuates lashes" , "Makeup used to darken and thicken the eye lashes" .)