Immobilised outright when hit (6)

I believe the answer is:

struck

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

I don't understand the remainder of the clue.

(Other definitions for struck that I've seen before include "hit hard" , "Hit; sounded (hour)" , "Found" , "Collided" , "Made to pause by something unexpected" .)

I've seen this clue in the Evening Standard.

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