Put one foot in front of the other (4)
I believe the answer is:
step
'put one foot in front of the other' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for step that I've seen before include "Section of flight" , "Movement in dancing" , "'Stage, rung (4)'" , "Place foot on as with a pest" , "Gradation" .)