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' is the 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" .)