"His time at Chelsea was divided between loan spells at AC Milan and cross town rivals Inter."
Here, "spell" is used to mean a period of time.
Footballers who are contracted to a club are sometimes loaned out to other clubs for a specific period, this is often referred to as a "loan spell". In this case Crespo was signed to Chelsea but he had loan spells with both AC Milan and Inter Milan during his time with the London club.
not a teacher