Dunce misread addition and subtracted (5)
I believe the answer is:
idiot
'dunce' is the definition.
(I've seen this before)
'misread addition and subtracted' is the wordplay.
'misread' indicates anagramming the letters.
'subtracted' is a deletion indicator.
'addition' with 'and' taken out is 'ditio'.
'ditio' anagrammed gives 'IDIOT'.
(Other definitions for idiot that I've seen before include "Foolish person" , "Dolt" , "Dunce" , "Dipstick" , "Foolish character" .)