Once paid, it should be done (4)

I believe the answer is:

duty

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'it should be done' is the definition.
The definition suggests a singular noun which matches the answer.

I don't understand the remainder of the clue.

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(Other definitions for duty that I've seen before include "Cancel (a law)" , "Moral/legal obligation" , "Service - tax - obligation" , "Allegiance" , "Levy - tax on imports" .)

I've seen this clue in the Evening Standard.

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