Mostly gentle, a river that divides counties (5)

I believe the answer is:

tamar

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'river that divides counties' is the definition.
'tamar' can be an answer for 'river' (I've seen this before). I'm not certain of the remainder of the definition.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tamar that I've seen before include "West Country river; a tram (anag.)" , "Devon-Cornwall boundary river" , "Plymouth river" , "River of SW_England" , "River of Devon-Cornwall border" .)

I've seen this clue in The Telegraph.

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