Heartless opponent making money (4)
I believe the answer is:
rial
'money' is the definition.
(I've seen this before)
'heartless opponent' is the wordplay.
'heartless' suggests removing the centre.
'opponent' becomes 'rival' (a rival is an opponent or enemy).
'rival' with its centre removed is 'RIAL'.
'making' acts as a link.
(Other definitions for rial that I've seen before include "Bit of money" , "foreign money" , "Unit of money in Iran, like lira" , "Middle Eastern money" , "Cash" .)