Load of extra money but not a billion (4)
I believe the answer is:
onus
'load' is the definition.
('onus' can be a synonym of 'load')
'extra money but not a billion' is the wordplay.
'extra money' becomes 'bonus' (I've seen this before).
'but not' is a deletion indicator.
'a billion' becomes 'b'.
'bonus' with 'b' taken out is 'ONUS'.
'of' acts as a link.
(Other definitions for onus that I've seen before include "Liability" , "'Burden, of proof (4)'" , "Burden, duty" , "'Burden, load (4)'" , "'Responsibility, task or burden (4)'" .)