Begin a fall (4)
I believe the answer is:
trip
'begin a fall' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trip that I've seen before include "Journey, excursion" , "Stumble - excursion" , "often occurring just before the fall" , "Hallucinatory experience" , "Journey - miss one's footing" .)