Dive or fall headlong (6)
I believe the answer is:
plunge
I believe this clue is a double definition.
'dive' is the first definition.
(I know that dive can be written as plunge)
'fall headlong' is the second definition.
(I've seen this before)
(Other definitions for plunge that I've seen before include "Dive quickly" , "Dive - submerge" , "Drop steeply" , "Jump or dive into water" , "Dive or throw into" .)