What does the cryptic crossword clue "Muck, say, in unopened passage (4)" mean?

The definition is "Muck,". The wordplay uses a deletion. ISLE (deletion) = ISLE; definition: "Muck," The answer is ISLE.

Muck, say, in unopened passage (4)

Guardian Cryptic #29780 | 15 Across | 2025-08-22 flower