Agitation involving a part of flight (5)
I believe the answer is:
Here is my best explanation:
'part of flight' is the definition.
'agitation involving a' is the subsidiary indication.
'agitation' becomes 'stir' (in the sense of a disturbance).
'involving' indicates putting letters inside.
'stir' enclosing 'a' is 'stair'.
(Other definitions for stair that I've seen before include "Flight unit", "Tarsi (anag)", "Word linking way and well", "One of a flight of steps", "A step on the way up or down", "One of a 15", "One step in a series", "Step in house", "Step up or step down?" and "Step in a series".)