Crazy backing Knock Senseless? (4)

I believe the answer is:

stun

I'm a little stuck... Click here to teach me more about this clue!

'iknock senselessi?' is the definition.
I can't tell whether this defines the answer.

'crazy backing' is the wordplay.
'crazy' becomes 'nuts' (I've seen this before).
'backing' is a reversal indicator.
'nuts' back-to-front is 'STUN'.

Can you help me to learn more?

(Other definitions for stun that I've seen before include "Concuss; amaze" , "Daze, knock senseless" , "floor" , "Daze, stupify" , "Shock, flabbergast" .)

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

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