Screen Glen in auditorium (4)
I believe the answer is:
veil
'screen' is the definition.
(I've seen this before)
'glen in auditorium' is the wordplay.
'glen' becomes 'vale' (both can mean a type of valley).
'in auditorium' shows a homophone (sound like) (as the word might sound to an audience).
'vale' sounds like 'VEIL'.
(Other definitions for veil that I've seen before include "Cover, conceal" , "Nun's headdress" , "Conceal, screen" , "Transparent face cover" , "Concealing material" .)