Fish sketches in the auditorium (5)
I believe the answer is:
ruffs
'fish' is the definition.
'sketches in the auditorium' is the wordplay.
'sketches' becomes 'roughs'.
'in the auditorium' shows a homophone (sound like) (ie as heard on a stage).
'roughs' sounds like 'RUFFS'.
(Other definitions for ruffs that I've seen before include "Neck frills" , "More than one frill" , "changes suit for one's benefit" , "Elizabethan collars" .)