What does the cryptic crossword clue "Man on board eclipsing second most powerful Russian once (4)" mean?

The wordplay uses a container (one word inside another). S (synonym="second") in TAR (synonym="sailor, man on board") = TSAR The answer is TSAR.

Man on board eclipsing second most powerful Russian once (4)

Guardian Cryptic #29704 | 10 Across | 2025-05-26 flower