Be unsteady on one's feet (6)
I believe the answer is:
totter
'be unsteady on one's feet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for totter that I've seen before include "Waver and threaten to fall" , "Move unsteadily; be about to fall" , "I add" , "rock" , "Maybe a mathematician" .)