Hollow roar breaking boundaries when flying -- at this? (5,7)
I believe the answer is:
sound barrier
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'hollow roar breaking boundaries when flying' is the wordplay.
'hollow' means to remove the middle letters (the word is hollowed out).
'breaking' is an insertion indicator.
'when flying' is an anagram indicator (letters fly around).
'roar' with its middle taken out is 'rr'.
'boundaries' anagrammed gives 'soundbaier'.
'rr' put within 'soundbaier' is 'SOUND BARRIER'.
'at' is the link.
Can you help me to learn more?
(Other definitions for sound barrier that I've seen before include "Speed-related problem (for aircraft)" , "Air resistance" , "It is passed with a boom" .)