What does the cryptic crossword clue "Serb in UN quoting different key issue (7,8)" mean?

The wordplay uses an anagram. anagram of SERB + IN + UN + QUOTING = BURNINGQUESTION The answer is BURNINGQUESTION.

Serb in UN quoting different key issue (7,8)

Daily Mail Cryptic #17026 | 7 Down | 2023-02-03 flower