Advance payment had to include note (4)
I believe the answer is:
ante
'advance payment' is the definition.
(I've seen this in another clue)
'had to include note' is the wordplay.
'had' becomes 'ate' (e.g. I had an ice cream).
'to include' indicates putting letters inside.
'note' becomes 'n' (abbreviation).
'ate' enclosing 'n' is 'ANTE'.
(Other definitions for ante that I've seen before include "Player's stake in poker - a ten? (anag)" , "Advance stake" , "A neat sort of prefix" , "Poker pot starter" , "See what gambler risks" .)