Under-payment? (3)
I believe the answer is:
sub
I believe this clue is a double definition.
'under' is the first definition.
(I've seen this before)
'payment?' is the second definition.
(I have seen 'Advance payment ' mean 'sub' so perhaps 'payment' could also mean 'sub')
(Other definitions for sub that I've seen before include "Contribution; a boat" , "Underwater boat" , "Junior editor" , "Reserve (abbr)" , "Loan" .)