Hard to avoid surprise blow (4)

I believe the answer is:

sock

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'blow' is the definition.
(I have seen 'Hard blow ' mean 'sock' so perhaps 'blow' could also mean 'sock')

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for sock that I've seen before include "Thump" , "Item of footwear" , "Hit; horse's leg marking" , "Foot garment" , "component of Christmas present" .)

I've seen this clue in The Independent.

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