Sudden surprise missing hard blow (4)

I believe the answer is:

sock

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

I don't understand how the rest of the clue works.

(Other definitions for sock that I've seen before include "Wallop" , "Hit hard - footwear" , "component of Christmas present" , "Hit forcefully; footwear" , "Item of clothing" .)

I've seen this clue in The Sun.

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