Flight unit (5)
I believe the answer is:
stair
'flight unit' is the definition.
(I've seen this before)
'unit' is the wordplay.
I cannot quite understand how this works, but
'unit' could be 'air' (air unit is a kind of unit) and 'air' is found in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stair that I've seen before include "A step on the way up or down" , "one's seen in flight" , "One step in a series" , "Set of steps" , "means to go down" .)