What does the cryptic crossword clue "Rowdy boy in Merc not yet fully developed? (9)" mean?

The wordplay uses an anagram. anagram of BOY + IN + MERC = EMBRYONIC The answer is EMBRYONIC.

Rowdy boy in Merc not yet fully developed? (9)

Daily Mail Cryptic #17579 | 16 Down | 2025-03-20 flower