Some of the choir are lying — but not often (6)
I believe the answer is:
rarely
'not often' is the definition.
('rarely' is the opposite to 'often')
'some of the choir are lying' is the wordplay.
'some of' indicates a hidden word.
'RARELY' can be found hidden inside 'choir are lying'.
'but' acts as a link.
(Other definitions for rarely that I've seen before include "Hardly ever, most infrequently" , "Gas used in fluorescent signs" , "very well" , "Seldom, not frequently" , "once in a blue moon" .)