Agitation about one part of a flight (5)
I believe the answer is:
stair
'part of a flight' is the definition.
(I've seen this before)
'agitation about one' is the wordplay.
'agitation' becomes 'stir' (in the sense of a disturbance).
'about' means one lot of letters goes inside another.
'one' becomes 'a' (a thing is one thing).
'stir' placed around 'a' is 'STAIR'.
(Other definitions for stair that I've seen before include "Bit of a flight!" , "one on the way up" , "section of flight" , "one's seen in flight" , "One of a 15" .)