Duty officer (9)
I believe the answer is:
exciseman
'duty officer' is the definition.
The answer and definition can be both people as well as being singular nouns.
I also know that 'duty' relates to this answer.
'duty officer' is the wordplay.
'duty' becomes 'excise' (I've seen this in other clues).
'officer' becomes 'man' (I can't justify this - if you can you should give a lot more credence to this answer).
'excise'+'man'='EXCISEMAN'
(Other definitions for exciseman that I've seen before include "Old customs official" , "Duty collector (hist.)" , "He is given the duty" , "Old smuggler's foe" , "Burns, say" .)