Tragedy a 'thesp' actor performing (11)
I believe the answer is:
catastrophe
'tragedy' is the definition.
('catastrophe' can be a synonym of 'tragedy')
'a thesp actor performing' is the wordplay.
'performing' indicates anagramming the letters.
'a'+'thesp'+'actor'='athespactor'
'athespactor' with letters rearranged gives 'CATASTROPHE'.
(Other definitions for catastrophe that I've seen before include "final event" , "Calamity" , "disastrous outcome" , "Terrible event" , "Sudden disaster" .)