Instrument leaders of group used in the auditorium rarely (6)
I believe the answer is:
guitar
'instrument' is the definition.
(guitar is a kind of instrument)
'leaders of group used in the auditorium rarely' is the wordplay.
'leaders of' indicates taking the first letters.
The first letters of 'group used in the auditorium rarely' is 'GUITAR'.
(Other definitions for guitar that I've seen before include "Fretted string instrument" , "Stringed instrument often played by pop groups" , "Stringed instrument, may be electric" , "One played with plectrum" , "Axe" .)