River — it may mark the spot between two points (3)

I believe the answer is:

exe

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'river' is the definition.
(River Exe)

I don't understand the rest of the clue.

(Other definitions for exe that I've seen before include "Topsham's river" , "Somerset/Devon river" , "River of SW England" , "Runner" , "Tiverton's river" .)

I've seen this clue in The Times.

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