First spent, extra cash for duty (4)
I believe the answer is:
onus
'for duty' is the definition.
(I've seen this in another clue)
'first spent extra' is the wordplay.
'first spent' means to remove the first letter (the first letter is wasted or spent).
'extra' becomes 'bonus' (I've seen this before).
'bonus' with its first letter removed is 'ONUS'.
'cash' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for onus that I've seen before include "Duty or responsibility" , "Obligation - charge" , "'Burden, of proof (4)'" , "'Responsibility, task or burden (4)'" , "Weight of responsibility" .)