It's wet at Oxford (4)

I believe the answer is:

isis

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'oxford' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for isis that I've seen before include "Sister of Osiris" , "Oxfordshire river" , "Exists twice for a goddess" , "Repetitive Egyptian goddess" , "The upper Thames" .)

I've seen this clue in the Evening Standard.

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