Crossword Genius

Don't loaf around so much, it being dark (8)

Ross

I believe the answer is:

moonless

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

'dark' is the definition.
(I know that moonless is a more specific form of the attribute dark)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Guardian.
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