Lose one's footing (4)
I believe the answer is:
slip
'lose one's footing' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for slip that I've seen before include "failure" , "Slide; petticoat" , "Lose footing as on banana skin" , "Silly little mistake" , "Lingerie item - fielding position" .)