Agitation involving a part of flight (5)
I believe the answer is:
stair
'part of flight' is the definition.
(I've seen this before)
'agitation involving a' is the wordplay.
'agitation' becomes 'stir' (stir is a kind of agitation).
'involving' is an insertion indicator.
'stir' placed around 'a' is 'STAIR'.
(Other definitions for stair that I've seen before include "Step as part of flight" , "that's between levels" , "section of flight" , "means to go down" , "Rising step" .)