Boater perhaps striking the second a reversal frustrates (7)

I believe the answer is:

thwarts

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'frustrates' is the definition.
('thwart' can be a synonym of 'frustrate')

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

(Other definitions for thwarts that I've seen before include "Blocks, prevents" , "Frustrates" .)

I've seen this clue in The Daily Express.

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