The narrator is telling a story about seventeen oranges. The boy or the young man was working in the docks as a cart driver. He had the habit of taking things that came in the ship and putting it in his apron. Clem, another friend of his, also took things that way, and by chance, was not caught.
Once, a police officer named Pongo caught the boy for stealing seventeen oranges. Pongo locked the boy up in the cabin to bring a witness. Pongo and his friend, another police officer, came to see that the seventeen oranges which were stolen by the boy, but it was gone.
Pongo - the policeman
Pongo could not believe that a boy could eat seventeen oranges with their peels and seeds. But the boy did that to erase the evidence and not to get into prison.