Be on the brink (6)
I believe the answer is:
'be on the brink' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for teeter that I've seen before include "Wobble as though about to fall" , "Totter, wobble" , "Sway back and forth on the edge" , "Sway back and forth, on the brink perhaps" , "See-saw on the edge" .)