What does the cryptic crossword clue "Stomachs turned by dirty joke (4)" mean?

The definition is "dirty joke". The wordplay uses a reversal. TUMS (reversal="”Stomachs”") = SMUT; definition: "dirty joke" The answer is SMUT.

Stomachs turned by dirty joke (4)

Guardian Cryptic #29121 | 10 Across | 2023-07-13 flower