Check an oarsman's support (6)

I believe the answer is:

thwart

This clue is a double definition.

'check' is the first definition.
(to check is to frustrate or spoil a plan)

'an oarsman's support' is the second definition.
(a thwart is a seat in a rowing boat)

(Other definitions for thwart that I've seen before include "Stymie" , "Snooker" , "Frustrate, prevent from doing" , "check" , "Prevent from accomplishing something" .)

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

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