Left theatrical role in middle of act (8)
I believe the answer is:
departed
'left' is the definition.
(departing is a kind of leaving)
'theatrical role in middle of act' is the wordplay.
'theatrical role' becomes 'part' ('part' can be a synonym of 'theatrical role').
'in middle of' means one lot of letters goes inside another.
'act' becomes 'deed' (deed is a kind of act).
'part' placed into 'deed' is 'DEPARTED'.
(Other definitions for departed that I've seen before include "completely breathless" , "Went - dead" , "Gone, maybe dead" , "Left -- late" , "Dead; set off" .)