Idiot returned money with reduction (6)
I believe the answer is:
nitwit
'idiot' is the definition.
(I've seen this before)
'returned money with reduction' is the wordplay.
'returned' is a reversal indicator.
'money' becomes 'tin' (I've seen this before**).
'reduction' means to remove the last letter.
'tin' backwards is 'nit'.
'with' with its last letter taken away is 'wit'.
'nit'+'wit'='NITWIT'
(Other definitions for nitwit that I've seen before include "Blockhead" , "Foolish person like humorous louse" , "Turkey" , "Silly person (colloq.)" , "Silly fool" .)