It's instrumental in making a moving stair (5)
I believe the answer is:
sitar
'it's instrumental' is the definition.
(sitar is a kind of musical instrument)
'moving stair' is the wordplay.
'moving' indicates an anagram.
'stair' anagrammed gives 'SITAR'.
'in making a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sitar that I've seen before include "Long necked and often plucked" , "Hindu stringed instrument" , "music producer" , "Indian stringed instrument" , "Large Indian lute" .)