What does the cryptic crossword clue "Corpulence in fast set when in a loose state (9)" mean?

The wordplay uses an anagram. anagram of IN + FAST + SET = FATTINESS The answer is FATTINESS.

Corpulence in fast set when in a loose state (9)

Daily Mail Cryptic #17275 | 14 Down | 2024-01-19 flower