Some light pen confiscated from lusty hombres (5)

I believe the answer is:

lumen

I'm a little stuck... Click here to teach me more about this clue!

'some light' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for lumen that I've seen before include "Unit of luminous flux" , "Unit of light flux" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.