Flawless defence yet speedily broken (5,7)
I believe the answer is:
sound barrier
'flawless defence' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sound barrier that I've seen before include "Speed-related problem (for aircraft)" , "It is passed with a boom" , "Air resistance" .)