Impact one expects to have? (4)

I believe the answer is:

bump

'impact' is the definition.
(bump is a kind of impact)

I can't explain the remainder of the clue.

(Other definitions for bump that I've seen before include "Light blow or jolting collision" , "Impact injury" , "Swelling caused by a blow perhaps" , "Lump caused by a blow" , "Jolt - collision - swelling" .)

I've seen this clue in The Times and The Sun.

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