A blow with an oar (6)

I believe the answer is:

stroke

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'a blow' is the definition.
(I've seen this before)

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

(Other definitions for stroke that I've seen before include "breast, for one" , "Blow ?- single movement" , "Caress gently" , "Caress; swimming style" , "Stoker (anag.)" .)

I've seen this clue in the Evening Standard.

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