One singer in a trio? (6)

I believe the answer is:

treble

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'one 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 "Two-channel sound" , "Inner ring score (darts)" , "a very large one" , "Very big drink" , "high frequency" .)

I've seen this clue in the Evening Standard.

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