What does the cryptic crossword clue "Four start to alter an artificial surface (9)" mean?

The definition is "surface". The wordplay uses an anagram. anagram of FOUR + START = ASTROTURF; definition: "surface" The answer is ASTROTURF.

Four start to alter an artificial surface (9)

Daily Mail Cryptic #17328 | 7 Down | 2024-04-03 flower