for him to tell pointless lies about his supposed continued desire for a clerical life? It's not really relevant to his real purpose of trying to find out how much she has discovered about his past, yet he raises it unnecessarily. One might think he would avoid the subject, since anything Elizabeth had heard could not possibly be to Wickham's credit - as indeed she lets him know, subtly. Possibly I have answered my own question - Wickham is indeed compelled to lie, by his author, who wants to give her heroine the chance to slap him down, metaphorically speaking.