Additional payment maybe in currency unit one has (9)
I believe the answer is:
incentive
'additional payment' is the definition.
'incentive' can be an answer for 'payment' (incentive is a kind of payment). I'm not certain of the 'additional' bit.
'in currency unit one has' is the wordplay.
'currency unit' becomes 'cent' (I can't explain this - if you can you should give a lot more credence to this answer).
'one has' becomes 'I've' ('one has' can formally or humorously mean 'I have').
'in'+'cent'+'ive'='INCENTIVE'
'maybe' is the link.
Can you help me to learn more?
(Other definitions for incentive that I've seen before include "Motivational reward" , "encouragement" , "carrot" , "Stimulus" , "Payment to improve" .)