Formerly being on the stage was strenuous (8)
I believe the answer is:
exacting
'strenuous' is the definition.
(thesaurus)
'formerly being on the stage' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'being on the stage' becomes 'acting' (I've seen this before).
'ex'+'acting'='EXACTING'
'was' is the link.
(Other definitions for exacting that I've seen before include "Demanding much" , "Greatly demanding" , "Strict, demanding" , "Stringent" , "Making great demands like difficult task" .)