They make good stuff that keeps theatre ventilated (9)
I believe the answer is:
'they make good stuff that' is the definition.
'repairers' can be an answer for 'they make good' (to make good is to fix or repair). I am unsure of the rest of the definition.
'theatre ventilated' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'ventilated' becomes 'airers' (I can't explain this - if you can you should believe this answer much more).
'keeps' is the link.