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