Singer in a trio? (6)

I believe the answer is:

treble

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'singer' is the definition.
(I've seen this in other clues)

I can't explain the rest of the clue.

(Other definitions for treble that I've seen before include "Feat of winning three major trophies" , "Multiply by three - high-pitched" , "Multiple score in darts" , "Threefold - singing voice" , "Triple - soprano voice" .)

I've seen this clue in the Evening Standard.

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