Waiter drops one drink (5)
I believe the answer is:
water
'drink' is the definition.
(water is an example)
'waiter drops one' is the wordplay.
'drops' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'waiter' with 'i' taken away is 'WATER'.
(Other definitions for water that I've seen before include "Classical element" , "Essential liquid" , "Irrigate" , "It's to be found in puddles, pools and ponds" , "Commonest fluid" .)