What does the cryptic crossword clue "Regular twinkling mostly seen in stone (6)" mean?

The wordplay uses a container (one word inside another). TRIC (synonym="TRIC") in ST (synonym="stone, unit of mass") = STRICT The answer is STRICT.

Regular twinkling mostly seen in stone (6)

Guardian Cryptic #29517 | 22 Down | 2024-10-18 flower