I had nothing invested in it, being a fool (5)
I believe the answer is:
idiot
'being a fool' is the definition.
(I've seen this before)
'i had nothing invested in it' is the wordplay.
'i had' becomes 'id' (I'd).
'nothing' becomes 'o' (looks like zero - 0).
'invested in' is an insertion indicator.
'o' placed into 'it' is 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Village character" , "Imbecile" , "Dunce" , "Ass, nincompoop" , "Fool, silly person" .)