Crazy Horse enters wide circle (6)

I believe the answer is:

whacko

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'crazy' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'horse enters wide circle' is the wordplay.
'horse' becomes 'hack' (hack is a kind of horse).
'enters' indicates putting letters inside.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'circle' becomes 'O' (O is a circle).
'w'+'o'='wo'
'hack' placed within 'wo' is 'WHACKO'.

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(Other definitions for whacko that I've seen before include "Jolly good" , "Mad" , "Eccentric" , "nuts" .)

I've seen this clue in The Times.

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