Conscription is a duty (4)
I believe the answer is:
levy
This clue is a double definition.
'conscription' is the first definition.
(levy is a kind of conscription)
'duty' is the second definition.
(I've seen this before)
'is a' is the link.
(Other definitions for levy that I've seen before include "Tax, fee, fine, etc" , "enlisted troops" , "tax money" , "Imposition of charge" , "Tax raised" .)