Treacherously water-borne but neither English nor European pest (5,3)

I believe the answer is:

brown rat

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'pest' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the rest of the clue.

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(Another definition for brown rat that I've seen is " Common rodent pest".)

I've seen this clue in The Telegraph.

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