Crossword Genius

Unreasonable dunce would be this (1,3,5)

Ross

I believe the answer is:

a bit thick

'unreasonable' is the definition.
(I've seen this before)

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

(Other definitions for a bit thick that I've seen before include "over the top" , "none too bright!" , "more than one should have to bear" , "Somewhat lacking in intelligence" , "Unreasonable or unfair - somewhat obtuse" .)

I've seen this clue in The Guardian.
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