What does the cryptic crossword clue "Essential wear first and foremost for Shrek character (6)" mean?

The wordplay uses a charade (building blocks). DON (synonym="wear") + KEY (synonym="essential") = DONKEY The answer is DONKEY.

Essential wear first and foremost for Shrek character (6)

Guardian Cryptic #29233 | 19 Down | 2023-11-21 flower