In the novel Postern of Fate, Clarence is a friend of Henry Bodlicott. He knows about the Beresford's exploits in N or M? and is very pleased they have come to live in Hollowquay. Unlike many others, he listened with interest to the stories which Old Isaac, Henry's grandfather, had to tell about the old days, especially the events at Swallow's Nest. With all the mentions of secret meetings and hiding places, Clarence told Henry "it's just like in a film."
When Tuppence formed Henry's friends into an informal junior brigade to help, Clarence emerges as the informal leader. He gets the most action and paragraph bandwidth and his contributions are probably the most useful. At one point he tells the others, "It's my show...." He comes up with the idea that Tuppence should visit the Pensioners' Palace Club to listen in on old folks' gossip and is one of three picked to escort Tuppence there. Later, he comes on his own to help explore the inside of the KK greenhouse. Clarence used to help Old Isaac in the garden and he knew, for example, where the key to KK was kept. He also knew how to open up the porcelain stool named "Cambridge". Apparently it could be unsrewed after being turned upside down, so that the inside can be revealed without having to break the stool.