In a hospital, it's part of the theatre (3)
I believe the answer is:
pit
'part of the theatre' is the definition.
('pit' is part of 'theatre')
'in a hospital it's' is the wordplay.
'in' says the answer is hidden in the clue.
'PIT' is hidden in the letters of 'hospital ts'.
(Other definitions for pit that I've seen before include "Coalmine" , "Take stone" , "Hellhole?" , "Tip into crater" , "Cherry stone" .)