What does the cryptic crossword clue "In the main it’s a complicated allergy treatment (13)" mean?

The wordplay uses an anagram. anagram of IN + THE + MAIN + IT’S + A = ANTIHISTAMINE The answer is ANTIHISTAMINE.

In the main it’s a complicated allergy treatment (13)

Daily Mail Cryptic #16504 | 19 Across | 2021-02-03 flower