Musical composition person at auditorium includes (6)
I believe the answer is:
sonata
'musical composition' is the definition.
(I've seen this before)
'person at auditorium includes' is the wordplay.
'includes' indicates the answer is hidden within the clue.
'SONATA' can be found hidden inside 'person at auditorium'.
(Other definitions for sonata that I've seen before include "A musical form" , "Sat on a kind of composition" , "Piece of music for solo piano" , "Instrumental work" , "Musical work for a solo instrument" .)