It's up to him to see the government gets its due (3,9)
I believe the answer is:
tax collector
'it's up to him to see the government' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'its due' is the wordplay.
I cannot really understand how this works, but
'due' could be 'collect' (similar in meaning) and 'collect' is found within the answer.
The remaining letters 'taxor' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'gets' is the link.
Can you help me to learn more?
(Another definition for tax collector that I've seen is " Old-fashioned publican".)