Not trebles, but they've been known to sing in a trio (6)
I believe the answer is:
tenors
'trebles but they've been known to sing in a trio' is the definition.
I can't judge whether this definition defines the answer.
'not' is the wordplay.
I cannot really see how this works, but
'not' could be 'no' (negative) and 'no' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tenors that I've seen before include "Male singers with high register" , "Opera singers" , "Male singing voices" , "Male singers from Reston" .)