Does he make the apparatus in the theatre work? (8)
I believe the answer is:
operator
'does he make the apparatus' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'the theatre work?' is the wordplay.
'the theatre' becomes 'opera' (opera is a kind of theatre).
'work?' becomes 'tor' (I can't justify this - if you can you should give a lot more credence to this answer).
'opera'+'tor'='OPERATOR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for operator that I've seen before include "Agent that runs some apparatus or machine" , "One using an item of equipment" , "Person working a switchboard" , "Person at switchboard, etc" , "Person who runs a machine or a business" .)