COMMENTARY on 12:17
A. Yusuf Ali:

Translation:
They said: "O our father! We went racing with one another, 1649 and left Joseph with our things; and the wolf devoured him But you wilt never believe us even though we tell the truth." 1650
Commentary:

1649  They wanted to make out that they were not negligent of Joseph. They were naturally having games and exercise, while the boy was left with their belongings. It was the racing that prevented them from seeing the wolf. And Jacob's fears about the wolf ( 12:13 above) made them imagine that he would swallow the wolf story readily.

1650  They were surprised that Jacob received the story about the wolf with cold incredulity. So they grew petulant, put on an air of injured innocence, and brought out the blood-stained garment described in the next verse.