What does the cryptic crossword clue "Some 'yep', only on the contrary? (4)" mean?

The definition is "S". The wordplay uses a hidden word. Hidden reversed in "some 'yEP', ONly on the contrary?" The answer is NOPE.

Some 'yep', only on the contrary? (4)

Guardian Cryptic #29188 | 6 Down | 2023-09-29 flower