SW English cross-country runner (5)

I believe the answer is:

tamar

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'runner' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for tamar that I've seen before include "River of SW_England" , "River in south-west" , "River of SW England" , "woman who posed as a prostitute" , "River of Devon-Cornwall border" .)

I've seen this clue in the Evening Standard.

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