Concern about a race round area that is really crazy (3,2,1,6)

I believe the answer is:

mad as a hatter

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'crazy' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for mad as a hatter that I've seen before include "Completely daft" , "Crazy" , "Completely insane (informal)" , "Sun-orbiting body" , "Totally daft" .)

I've seen this clue in The Times.

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