When I leave the cab without paying, there's just no accounting! (3,4)

I believe the answer is:

tax free

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'when i' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

I don't understand the rest of the clue.

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(Other definitions for tax free that I've seen before include "relieved of all duties" , "showing no sign of duty" .)

I've seen this clue in the Evening Standard.

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