Dunce, I had nothing invested in it (5)
I believe the answer is:
idiot
'dunce' is the definition.
(I've seen this before)
'i had nothing invested in it' is the wordplay.
'i had' becomes 'I'd' (contraction).
'nothing' becomes 'O' (looks like zero - 0).
'invested in' means one lot of letters goes inside another.
'o' placed inside 'it' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Goose" , "Oaf" , "Clot" , "Blockhead" , "Fool, silly person" .)