Crossword Genius

The result of not thinning out - sheer stupidity (9)

Ross

I believe the answer is:

denseness

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

'stupidity' is the definition.
(I know that denseness is a type of stupidity)

I can't explain the rest of the clue.

Can you help me to learn more?

(Another definition for denseness that I've seen is " Stupidity".)

I've seen this clue in the Evening Standard.
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