What does the cryptic crossword clue "Dress up as Satan, passing masks on (8)" mean?

The wordplay uses a container (one word inside another). ON (synonym="ON") in DEMISE (synonym="death, passing") = DEMONISE The answer is DEMONISE.

Dress up as Satan, passing masks on (8)

Guardian Cryptic #29217 | 8 Across | 2023-11-02 flower