A reduced payment (3)

I believe the answer is:

sub

I'm a little stuck... Click here to teach me more about this clue!

'payment' is the definition.
(I have seen 'Advance payment ' mean 'sub' so perhaps 'payment' could also mean 'sub')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sub that I've seen before include "Replacement" , "Loan" , "Advance payment" , "Underwater boat" , "Vehicle in water" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app