Crossword Genius

American mineral water in Barking but not Washington! (5)

Ross

I believe the answer is:

niter

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

'american mineral' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

I've seen this clue in The Guardian.
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