Put one's foot down as a matter of duty (5)
I believe the answer is:
stamp
I believe this clue is a double definition.
'put one's foot down' is the first definition.
'a matter of duty' is the second definition.
(stamp duty is a tax)
'as' is the link.
(Other definitions for stamp that I've seen before include "Mark of approval" , "What to put on 7 before putting in 15" , "making an impression?" , "Postal token" , "Official mark; bang (foot)" .)