Crossword Genius

Flight unit (5)

Ross

I believe the answer is:

stair

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Penny Press, the Evening Standard, the King Feature Syndicate and The Wall Street Journal.
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