Attendant spills first drink (5)
I believe the answer is:
water
'drink' is the definition.
(I know that water is a type of drink)
'attendant spills first' is the wordplay.
'attendant' becomes 'waiter' (a waiter attends to diners).
'spills' indicates named letters should be taken away (some letters spill out).
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'waiter' with 'i' taken out is 'WATER'.
(Other definitions for water that I've seen before include "Transparent liquid" , "Adam's ale" , "Common fluid" , "Earth, air, fire partner" , "Commonest liquid" .)