Threshold of a boom in air travel (5,7)
I believe the answer is:
sound barrier
'air travel' is the definition.
The definition and answer can be both types of natural phenomena as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'threshold of a boom' is the wordplay.
'threshold of' becomes 'sound' (I am not sure about this - if you are sure you should believe this answer much more).
'a boom' becomes 'barrier' (I've seen this before).
'sound'+'barrier'='SOUND BARRIER'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for sound barrier that I've seen before include "Air resistance" , "Speed-related problem (for aircraft)" , "It is passed with a boom" .)