Be about to fall (6)
I believe the answer is:
teeter
'be about to fall' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for teeter that I've seen before include "waver" , "Wobble as though about to fall" , "Rock gently on the edge" , "Move with little assurance" , "Vacillate" .)