Hideous sight I witnessed in the auditorium? (7)
I believe the answer is:
eyesore
'hideous sight' is the definition.
(term for an ugly building)
'i witnessed in the auditorium?' is the wordplay.
'i' becomes 'eye' (I've seen this in other clues).
'witnessed' becomes 'saw' ('see' can be a synonym of 'witness').
'in the auditorium?' shows a homophone (sound like) (ie as heard on a stage).
'saw' is a homophone of 'sore'.
'eye'+'sore'='EYESORE'
(Other definitions for eyesore that I've seen before include "pigsty" , "terrible prospect" , "Monstrosity" , "Horrible sight" , "A monstrous carbuncle" .)