They make good stuff that keeps theatre ventilated (9)

'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).
'rep'+'airers'='REPAIRERS'

'keeps' is the link.