The event you refer to happens in Chapter 39 of this immensely funny novel. Hank and Sir Sagramor have a tournament, and as they move to meet each other in their first "clash," Hank comments that he hears a large number of voices cheering on Sir Sagramor, but only one voice that supports him, which says: "Go it, slim Jim!" As he comments, it is clear he thinks this voice must come from Clarence and that he is deliberately trying to imitate the vernacular that Hank himself has taught Clarence, which is why he uses it now to show his support of Hank in tournament against Sir Sagramor:
It was an even bet that Clarence had procured that favour for me - and furnished the language, too.
Thus the quote you have identified is a voice of support for Hank in his fight against Sir Sagramor, but using the vernacular common to Hank and his world, that he has taken with him into Arthurian England and passed on to Clarence.