What does the cryptic crossword clue "Glum and sour caterer upset patron? (7,8)" mean?

The wordplay uses an anagram. anagram of GLUM + SOUR + CATERER = REGULARCUSTOMER The answer is REGULARCUSTOMER.

Glum and sour caterer upset patron? (7,8)

Daily Mail Cryptic #17095 | 6 Down | 2023-05-11 flower