Step on a ladder (4)
I believe the answer is:
rung
'step on a ladder' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rung that I've seen before include "Tolled - step on ladder" , "Phoned" , "Level in a hierarchy" , "Step up the ladder - called for" , "Part of a ladder" .)