Voice of a boy of three? (6)

I believe the answer is:

treble

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'voice' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for treble that I've seen before include "Score of sixty with one dart?" , "Threefold; boy's voice" , "What boys sing" , "Triple - high-pitched" , "Boy singer" .)

I've seen this clue in the Evening Standard.

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