Quality, one not observed in restaurant worker (5)
I believe the answer is:
water
'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" .)