What does the cryptic crossword clue "Colleague and rival (not half!) (4)" mean?

The definition is "Colleague". The wordplay uses a reversal. reverse of OPPO = OPPO; definition: "Colleague" The answer is OPPO.

Colleague and rival (not half!) (4)

Daily Mail Cryptic #17252 | 12 Across | 2023-12-18 flower