What does the cryptic crossword clue "Rocky substrate's most difficult to penetrate (10)" 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.

Rocky substrate's most difficult to penetrate (10)

Times Cryptic #29424 | 26 Across | 2025-12-27 flower

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