Maybe sommelier's forgotten one drink (5)
I believe the answer is:
water
'drink' is the definition.
(I've seen this before)
'maybe sommelier's forgotten one' is the wordplay.
I cannot quite see 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 understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for water that I've seen before include "H20" , "Superior's full of it" , "Classical element" , "What can produce solution" , "Basic liquid" .)