There's a boom when it goes over one (4,6)
I believe the answer is:
mach number
'it goes over one' is the definition.
The definition suggests a singular noun which matches the answer.
'there's a boom when' is the wordplay.
I cannot quite understand how this works, but
'there's' could be 'n' (I've seen this in other clues) and 'n' is found within the answer.
'a' is found in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for mach number that I've seen is " Speed".)