Character, in middle of act, withdrew (8)
I believe the answer is:
departed
'withdrew' is the definition.
'character in middle of act' is the wordplay.
'character' becomes 'part' (synonyms).
'in middle of' indicates putting letters inside.
'act' becomes 'deed' (deed is a kind of act).
'part' put inside 'deed' is 'DEPARTED'.
(Other definitions for departed that I've seen before include "Dead; set off" , "Left -- late" , "Went - dead" , "Gone, maybe dead" , "completely breathless" .)