Rustic's family deserts him - what a blow! (4)

I believe the answer is:

bump

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'what a blow' is the definition.
(synonyms)

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

(Other definitions for bump that I've seen before include "Small swelling" , "Evidence of pregnancy - knock (into)" , "Speed restriction" , "striking" , "Prang" .)

I've seen this clue in The Globe and Mail.

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