Sort of bed you'll get in the Thames valley (5)

I believe the answer is:

river

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'valley' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for river that I've seen before include "One splits" , "Natural flow of water" , "Flowing water" , "Tributary" , "flower" .)

I've seen this clue in the Evening Standard.

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