It strikes one as a mild expletive (4)

I believe the answer is:

blow

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'it strikes one as a mild expletive' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

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(Other definitions for blow that I've seen before include "Powerful stroke; completely bungle (an opportunity)" , "Setback" , "haymaker, perhaps" , "Shock; sound (a horn)" , "Hit - disappointment" .)

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

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