Threaten to topple (6)
I believe the answer is:
teeter
'threaten to topple' 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 "Balance unsteadily" , "Rock precariously on the edge" , "Move with little assurance" , "See-saw on the edge" , "Rock gently on the edge" .)