Evan's abroad - it's OK in Bordeaux where the claret flows (4,4)

I believe the answer is:

vena cava

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'where the claret flows' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'evan's abroad it's ok in bordeaux' is the wordplay.
'abroad' indicates anagramming the letters.
'it's ok in bordeaux' becomes 'cava' (I can't explain this - if you can you should believe this answer much more).
'evan' is an anagram of 'vena'.
'vena'+'cava'='VENA CAVA'

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I've seen this clue in The Independent.

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