What does the cryptic crossword clue "Fine diet requires cooking? That's certain (8)" mean?

The definition is "certain". The wordplay uses an anagram. anagram of FINE + DIET = DEFINITE; definition: "certain" The answer is DEFINITE.

Fine diet requires cooking? That's certain (8)

Daily Mail Cryptic #17583 | 9 Across | 2025-03-26 flower