I've left the waiter something for a drink (5)
I believe the answer is:
water
'drink' is the definition.
(water is an example)
'i've left the waiter something for a' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'something' could be 'er' (I've seen this in other clues) and 'er' is located in the answer.
'a' is within the answer.
A single letter 'w' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for water that I've seen before include "Basic liquid" , "Compound of hydrogen and oxygen" , "What can produce solution" , "Diamond quality" , "Common fluid" .)