Transport notice encapsulated perverse hoax (5,4)

I believe the answer is:

mares nest

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'hoax' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for mares nest that I've seen before include "confused situation" , "muddle" , "It's disappointing" , "Complex and difficult situation" , "Chaotic situation" .)

I've seen this clue in the Sydney Morning Herald.

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