Demanding old flame once treading the boards (8)
I believe the answer is:
exacting
'demanding' is the definition.
(exacting is a kind of demanding)
'old flame once treading the boards' is the wordplay.
'old flame once' becomes 'ex'.
'treading the boards' becomes 'acting'.
'ex'+'acting'='EXACTING'
(Other definitions for exacting that I've seen before include "Meticulous" , "Difficult and onerous" , "Testing endurance or skill" , "Requiring great effort - sounds precise" , "Making great demands like difficult task" .)