A unit in flight (5)
I believe the answer is:
stair
This clue might be a double definition.
'a unit' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'flight' is the second definition.
(I've seen this before)
'in' is the link.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "Step as part of flight" , "that's between levels" , "one on the way up" , "single step in a series" , "Component of flight" .)