A flight unit? (5)
I believe the answer is:
stair
'a flight unit?' is the definition.
(I've seen this before)
'unit?' is the wordplay.
I cannot really 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 explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stair that I've seen before include "single step in a series" , "One step in a series" , "something of a case" , "Component of flight" , "one on the way up" .)