In order to enter a medieval university, students had to join a minor religious order of the Church. Once out of school, they were expected to seek secular employment outside the Church. The Cleric's books would have cost a small fortune (since they were still being copied by hand--no printing press yet). Chaucer gives the Cleric the stereotype of the starving student--both he and his horse are "thinner than a rake". He "found no preferment with the Church" yet he wasn't searching for secular employment, either. He seems content to borrow money from family and friends and spend it on books. He is studious, doesn't speak much, and doesn't put much stock in fancy clothing or things. He loved learning most, so teaching might be his chosen profession.
Chaucer's clerk, also called "the Oxforc cleric," is an older man who is wearing thin, raggedy clothes. His horse is very thin, and the clerk reads all of the time. He has never held a job, and he is constantly trying to find the secret of making gold so that he can be rich without having to work. He doesn't talk much, but he is respected when he does. He is very educated, but not very smart.
See the link below for an artist's interpretation of how the cleric may have looked. The most famous print version of The Canterbury Tales is the Ellesmere Manuscript, which contains many woodcut illustrations.