Fenland river rotates machinery (3)

I believe the answer is:

cam

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'fenland river' is the definition.
(River Cam)

I cannot understand how the remainder of the clue works.

(Other definitions for cam that I've seen before include "Rotating part in machine; river" , "Runner" , "Shaft on a rotating part of machine" , "academic stream" , "English river" .)

I've seen this clue in the Evening Standard.

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