Crossword Genius

A flight unit? (5)

Ross

I believe the answer is:

stair

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

'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" .)

Want a hint initially instead of a full solution? Install my app