Agitation involving a part of flight (5)
I believe the answer is:
Here is my best explanation:
'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 "One step of a flight", "One step in a series", "Way between floors", "Flight unit", "means to go down", "Tarsi (anag)", "Step in a series", "Indoor step", "one on the way up" and "Bit of a flight!".)