What does the cryptic crossword clue "Union campaigner messing up gets the bird (8,7)" mean?

The wordplay uses an anagram. anagram of UNION CAMPAIGNER = MACARONIPENGUIN The answer is MACARONIPENGUIN.

Union campaigner messing up gets the bird (8,7)

Guardian Cryptic #29210 | 1 Down | 2023-10-25 flower