Crossword Genius

Flawless defence yet speedily broken (5,7)

Ross

I believe the answer is:

sound barrier

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'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" .)

I've seen this clue in The Globe and Mail.
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