Register for a large drink (6)

I believe the answer is:

treble

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'for a large drink' is the definition.
(drink with three shots)

I cannot understand how the remainder of the clue works.

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(Other definitions for treble that I've seen before include "Choirboy" , "Threefold; boy's voice" , "Triple - soprano voice" , "Consisting of three parts; voice" , "What boys sing" .)

I've seen this clue in The Mirror.

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