Not very smart? That's rather unfair (1,3,5)
I believe the answer is:
a bit thick
'unfair' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for a bit thick that I've seen before include "Unreasonable or unfair - too broad?" , "Somewhat lacking in intelligence" , "too much!" , "Derogatory remark about waist" , "Tick habit off - it's quite unreasonable" .)