Multiply register (6)
I believe the answer is:
treble
I believe this clue is a double definition.
'multiply' is the first definition.
(I know that multiply is a is a more specific form of the action of treble)
'register' is the second definition.
(musical register)
(Other definitions for treble that I've seen before include "Two-channel sound" , "Hit in inner ring of dartboard" , "boy in church choir" , "Singer; in three parts" , "Inner ring score (darts)" .)