Server missing one essential liquid (5)
I believe the answer is:
water
'essential liquid' is the definition.
(I've seen this before)
'server missing one' is the wordplay.
'server' becomes 'waiter' ('waiter' can be a synonym of 'server').
'missing' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'waiter' with 'i' taken out is 'WATER'.
(Other definitions for water that I've seen before include "Superior's full of it" , "Commonplace drink" , "H2O" , "Classical element" , "Common liquid" .)