Be on the brink of toppling (6)
I believe the answer is:
teeter
'be on the brink of toppling' 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 "Rock unsteadily on feet" , "Wobble as though about to fall" , "waver" , "Sway, as if about to fall" , "Sway back and forth, on the brink perhaps" .)