The hole an orchestra may be in (3)
I believe the answer is:
pit
'the hole' is the definition.
(a pit is a large hole)
'an orchestra may be in' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'orchestra may' becomes 'pt' (I can't justify this - if you can you should believe this answer much more).
'be in' is an insertion indicator.
'i' placed inside 'pt' is 'PIT'.
Can you help me to learn more?
(Other definitions for pit that I've seen before include "Tip into miners' shaft" , "Where the orchestra sits at an opera" , "Tip into crater" , "Mine-shaft" , "Deep hole (in theatre?)" .)