A sex offender with adult victims, i.e. a rapist. The term "NONCE" doesn't necessarily mean child victims. It originated from Wakefield Prison in the UK and stood for "Not on Normal Courtyard Exercise." If a prisoner had committed horrific crimes such as child murder, rape or pedophilia they would get a "NONCE" sign on the door of their cell. This meant that the prison guards would not open their cell doors at the same time as the other prisons because they were very likely to get beaten up by the other inmates. It doesn't necessarily mean pedophiles.
An adult nonce is essentially a sex offender who has victims that are adults. For example a serial rapist or woman murderer but the victims are adults.
Wayne Couzens, the policeman who abused his position to rape and kill Sarah Everard is an adult nonce.