Check on oarsman's support (6)
I believe the answer is:
thwart
I believe this is a double definition.
'check' is the first definition.
(I've seen this before)
'oarsman's support' is the second definition.
(a thwart is a seat in a rowing boat)
'on' is the link.
(Other definitions for thwart that I've seen before include "Defeat" , "Oppose successfully" , "Snooker" , "Prevent from accomplishing something" , "Frustrate or foil" .)