Extra money collected by painter (estimated) (8)
I believe the answer is:
interest
'extra money' is the definition.
(eg extra money on a debt)
'collected by painter estimated' is the wordplay.
'collected by' indicates the answer is hidden within the clue.
'INTEREST' is hidden in the letters of 'painter estimated'.
(Other definitions for interest that I've seen before include "thing" , "Sense of concern or curiosity" , "Stake, share" , "Money paid on a loan" , "Money found in street" .)