In the theatre, it's next to the piano (3)
I believe the answer is:
pit
'in the theatre' is the definition.
I can't tell whether this definition defines the answer.
'it's next to the piano' is the wordplay.
'next to' is a charade indicator (letters next to each other).
'the piano' becomes 'p' (musical abbreviation).
'it' after 'p' is 'PIT'.
Can you help me to learn more?
(Other definitions for pit that I've seen before include "Where racing cars refuel" , "Mining hole" , "Coal mine" , "See 9" , "Mine; fruit stone" .)