Mad to retire, which does surprise you (4)

I believe the answer is:

stun

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'does surprise you' is the definition.
I can't judge whether this defines the answer.

'mad to retire' is the wordplay.
'mad' becomes 'nuts' (I've seen this before).
'to retire' is a reversal indicator (retire can mean to go back).
'nuts' back-to-front is 'STUN'.

'which' is the link.

Can you help me to learn more?

(Other definitions for stun that I've seen before include "Make senseless or dizzy with a blow" , "Daze, paralyse" , "Confound" , "Amaze; knock senseless" , "Nuts (anag)" .)

I've seen this clue in the Evening Standard.

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