One is vertical in flight (5)
I believe the answer is:
riser
This could be a double definition.
'one' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'vertical in flight' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'is' acts as a link.
Can you help me to learn more?
(Other definitions for riser that I've seen before include "One getting up" , "upright portion of step" , "Front of a step; vertical pipe" , "an essential to flight" , "Person getting out of bed" .)