Register for large drink (6)
I believe the answer is:
treble
I believe this is a double definition.
'register' is the first definition.
(musical register)
'large drink' is the second definition.
(drink with three shots)
'for' acts as a link.
(Other definitions for treble that I've seen before include "Hit in inner ring of dartboard" , "Boy chorister" , "High voice; occurring three times" , "Very big drink" , "High darts score" .)