Omen is unfinished by director retiring in March (4)
I believe the answer is:
demo
'march' is the definition.
(I've seen this before)
'omen is unfinished by director retiring' is the wordplay.
'is unfinished' means to remove the last letter.
'by' says to put letters next to each other.
'director' becomes 'd'.
'retiring' shows that the letters should be reversed in order.
'omen' with its last letter taken away is 'ome'.
'ome'+'d'='omed'
'omed' backwards is 'DEMO'.
'in' acts as a link.
(Other definitions for demo that I've seen before include "March (abbr)" , "exhibit" , "illustration" , "Brass instrument valve" , "Public protest meeting" .)