An obligation some people avoid by running! (4)

I believe the answer is:

duty

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'an obligation' is the definition.
(I know that obligation can be written as duty)

I can't explain the remainder of the clue.

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(Other definitions for duty that I've seen before include "Cancel (a law)" , "Official obligation" , "charge" , "Service - tax - office" , "Tax - burden" .)

I've seen this clue in the Evening Standard.

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