"Prosai-comi-epic writing "

Fielding is describing his novel as a comic epic written in prose.  An epic generally follows the triumphs and travails of a heroic character, which Tom is, to some extent. However, he’s a roguish hero, and not immune from blunder and embarrassment. This, together with the antics of courser characters such as Partridge and Squire Western, ensures the comedic quality of the tale.