Crossword Genius

Quality, one not observed in restaurant worker (5)

Ross

I believe the answer is:

water

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

'quality' is the definition.
(eg the phrase 'finest water' meaning high quality)

'one not observed in restaurant worker' is the wordplay.
I cannot really understand how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
The remaining letters 'wat' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for water that I've seen before include "Diamond quality" , "It's to be found in puddles, pools and ponds" , "Quality of diamonds" , "Common fluid" , "Inundate" .)

I've seen this clue in The Independent.
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