It's responsible for a boom in flying (5,7)
I believe the answer is:
sound barrier
'flying' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it's responsible for a boom' is the wordplay.
'it's responsible for' becomes 'sound' (I can't justify this - if you can 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 "It is passed with a boom" , "Speed-related problem (for aircraft)" , "Air resistance" .)