What does the cryptic crossword clue "Nice build-up's mistakenly not given promotion? (12)" mean?

The wordplay uses an anagram. anagram of NICE + BUILD-UP'S = UNPUBLICISED The answer is UNPUBLICISED.

Nice build-up's mistakenly not given promotion? (12)

Daily Mail Cryptic #17341 | 1 Across | 2024-04-22 flower