Stair set in motion (5)
I believe the answer is:
astir
'in motion' is the definition.
(I've seen this in another clue)
'stair set' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'stair' is an anagram of 'ASTIR'.
(Other definitions for astir that I've seen before include "Awake and out of bed" , "This isn't still" , "In motion - out of bed" , "Up and about -- is art (anag)" , "Excitedly moving" .)