Stand on three legs, fall, and do over (6)
I believe the answer is:
tripod
'stand on three legs' is the definition.
(I've seen this before)
'fall and do over' is the wordplay.
'fall' becomes 'trip' (as in tripping over).
'and' is a charade indicator (letters next to each other).
'over' says the letters should be written backwards.
'do' backwards is 'od'.
'trip'+'od'='TRIPOD'
(Other definitions for tripod that I've seen before include "Support needed" , "Three-legged stand as for camera" , "Stand on three legs" , "Pay heed" , "Drop it (anag)" .)