Rung or tread (5)
I believe the answer is:
stair
I believe this clue is a double definition.
'rung' is the first definition.
(thesaurus)
'tread' is the second definition.
(thesaurus)
(Other definitions for stair that I've seen before include "Flight unit" , "something of a case" , "one's seen in flight" , "Rising step" , "One step of a flight" .)