Demanding old flame is working in the theatre (8)
I believe the answer is:
exacting
'demanding' is the definition.
(exacting is a kind of demanding)
'old flame is working in the theatre' is the wordplay.
I cannot really understand how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
'working' could be 'acting' ('act' can be a synonym of 'work') and 'acting' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for exacting that I've seen before include "Tough" , "Arduous and demanding" , "Making rigorous demands" , "Demanding much" , "Greatly demanding" .)