Suitable voice for a trio? (6)

I believe the answer is:

treble

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'suitable voice' is the definition.
'treble' can be an answer for 'voice' (I've seen this before). I'm not sure about the 'suitable' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for treble that I've seen before include "High-frequency (radio control knob)" , "high frequency" , "Inner ring score (darts)" , "Consisting of three parts; voice" , "Hit in inner ring of dartboard" .)

I've seen this clue in The Globe and Mail.

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