Register for a drink (6)
I believe the answer is:
treble
I believe this is a double definition.
'register' is the first definition.
(musical register)
'for a drink' is the second definition.
(Other definitions for treble that I've seen before include "High boy's singing voice" , "Feat of winning three major trophies" , "Two-channel sound" , "High voice - large drink" , "boy in church choir" .)