Stairs unit (4)
I believe the answer is:
step
'stairs unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for step that I've seen before include "Place foot on as with a pest" , "Put foot on a pest" , "Pace (taken one at a time?)" , "Abrupt change; pace" , "Part of a stairway" .)