Once in the theatre, he demands his money's-worth (7)
I believe the answer is:
exactor
'he demands his money's-worth' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'demands' relates to this answer.
'once in the theatre' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'in the theatre' becomes 'actor' (I am not sure about this - if you are sure you should believe this answer much more).
'ex'+'actor'='EXACTOR'
Can you help me to learn more?
(Other definitions for exactor that I've seen before include "Demanding person" , "One demanding and obtaining (something from someone)" , "demanding type" , "One forcing payment" , "Core tax (anag.)" .)