Server in restaurant leaving one something to drink (5)
I believe the answer is:
water
'something to drink' is the definition.
(water is a drink)
'server in restaurant leaving one' is the wordplay.
'server in restaurant' becomes 'waiter' (I've seen this before).
'leaving' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'waiter' with 'i' taken away is 'WATER'.
(Other definitions for water that I've seen before include "One of the four classical elements" , "Gem quality; fluid" , "something damp" , "H20" , "Adam's ale" .)