Some singers are content to teach, or used to be! (6)
I believe the answer is:
chorus
'some singers' is the definition.
(chorus can mean a group of singers)
'content to teach or used to be' is the wordplay.
'content to' indicates a hidden word.
'CHORUS' is hidden in the letters of 'teach or used be'.
'are' is the link.
(Other definitions for chorus that I've seen before include "Supporting singers and dancers" , "Organized group of singers" , "Some chanting" , "Pipe up in unison" , "It joins a soloist in parts of songs" .)