What does the cryptic crossword clue "Brainboxes managed to penetrate spy agency (6)" mean?

This cryptic clue uses wordplay to arrive at the answer. Visit the page for progressive hints — definition, wordplay type, and a full step-by-step explanation.

Brainboxes managed to penetrate spy agency (6)

Guardian Cryptic #29744 | 5 Down | 2025-07-11 flower

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