What does the cryptic crossword clue "Unknown mineral long ago (4)" mean?

The definition is "long ago". The wordplay uses a charade (building blocks). Y (synonym of "Unknown") + ORE (synonym of "mineral"); definition: "long ago" The answer is YORE.

Unknown mineral long ago (4)

Guardian Cryptic #27674 | 27 Across | 2018-11-23 flower

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