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 see.
This explanation may well be incorrect...
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" .)