Dunce Tees had nothing invested in it (5)
I believe the answer is:
idiot
'dunce' is the definition.
(I've seen this in another clue)
'tees had nothing invested in it' is the wordplay.
'tees had' becomes 'I'd' (where the crossword setter is 'Tees').
'nothing' becomes 'O' (O resembles 0).
'invested in' is an insertion indicator.
'o' going into 'it' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "'The -----'. novel by Dostoyevsky (1868)" , "Foolish character" , "Steal small amount" , "Blockhead" , "one needing head examined?" .)