It may be moving, possibly astir (5)
I believe the answer is:
stair
'it may be moving' is the definition.
The definition suggests a singular noun which matches the answer.
'possibly astir' is the wordplay.
'possibly' is an anagram indicator.
'astir' is an anagram of 'STAIR'.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "section of flight" , "One of a 15" , "this could get one down" , "Step in house" , "one on the way up" .)