In France, a water department (4)

I believe the answer is:

oise

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'department' is the definition.
(I have seen 'French department' mean 'oise' so perhaps 'department' could also mean 'oise')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for oise that I've seen before include "French department" , "part of France" .)

I've seen this clue in the Evening Standard.

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