What does the cryptic crossword clue "Agree treatment for main horse (9)" mean?

The definition is "Agree". The wordplay uses an anagram. anagram of MAIN + HORSE = HARMONISE; definition: "Agree" The answer is HARMONISE.

Agree treatment for main horse (9)

Daily Mail Cryptic #17385 | 16 Across | 2024-06-21 flower