What does the cryptic crossword clue "Perhaps Nicholas or Alexander's son nursed by Jack (4)" mean?

The wordplay uses a container (one word inside another). S (synonym="son") in TAR (synonym="jack") = TSAR The answer is TSAR.

Perhaps Nicholas or Alexander's son nursed by Jack (4)

Guardian Cryptic #29331 | 24 Down | 2024-03-15 flower