What does the cryptic crossword clue "Man, a GP, is in order in pre-Christian religion (8)" mean?

The wordplay uses an anagram. anagram of MAN, + A + GP, + IS = PAGANISM The answer is PAGANISM.

Man, a GP, is in order in pre-Christian religion (8)

Daily Mail Cryptic #17273 | 29 Across | 2024-01-17 flower