Thespian's role in act's vanished (8)
I believe the answer is:
departed
'vanished' is the definition.
(thesaurus)
'thespian's role in act's' is the wordplay.
'thespian's role' becomes 'part' (part in a play).
'in' is an insertion indicator.
'act' becomes 'deed' (deed is a kind of act).
'part' inserted into 'deed' is 'DEPARTED'.
(Other definitions for departed that I've seen before include "Quit" , "Went - dead" , "completely breathless" , "Gone, maybe dead" , "Left, went away" .)