It's singularly striking (4)

I believe the answer is:

oner

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But I can't explain it at all! Could you help me understand?

(Other definitions for oner that I've seen before include "Hit" , "Big blow" , "A PS1 note (colloq.)" , "One pound note; big lie" , "remarkable person" .)

I've seen this clue in the Evening Standard.

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