A French water department (5)

I believe the answer is:

meuse

I'm a little stuck... Click here to teach me more about this clue!

'a french water' is the definition.
'Meuse' can be an answer for 'water' (Meuse is an example). I'm not certain of the 'a french' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Another definition for meuse that I've seen is " Dept of NE France".)

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