Immediately payable (3)
I believe the answer is:
due
'immediately payable' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for due that I've seen before include "Owed, or payable immediately" , "Entitlement" , "Escort" , "...more than one there [ROME]" , "Expected; proper" .)