Aadjectival clause always includes a what

 The clause starts with a pronoun such as who, whom, that, or which or an adverb such as when, where and why.

The answer is "he was bad enough in all conscience, but the devil himself could not temp him to turn slave-trader. satirical is a synonym for sarcastic. so when the author says that tom walker could not be tempted by the devil he does not literally mean "the devil" but he means that he is too morally upright to become a slave-trader.