Shake a little, wanting medium-sized drink (6)
I believe the answer is:
treble
'drink' is the definition.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for treble that I've seen before include "winning series" , "Member of choir" , "Choirboy" , "Three times - a boy soprano" , "Feat of winning three major trophies" .)