Curse of all time, hapless leaders (4)
I believe the answer is:
oath
'curse' is the definition.
('oath' can be a synonym of 'curse')
'of all time hapless leaders' is the wordplay.
'leaders' says to take the initial letters.
The first letters of 'of all time hapless' is 'OATH'.
(Other definitions for oath that I've seen before include "Swear word" , "Curse or solemn word" , "bad language" , "Sworn promise" , "'Profane, or solemn expression (4)'" .)