It sounds like you have to identify a belief/perspective/assumption rather than constructing one of your own. Your teacher's use of the word "underlying" indicates that you are supposed to identify a belief/perspective/assumption that is not directly stated. Here are some questions to get you going:
Base on evidence from the text, how does Chinua Achebe feel about people who colonize other countries? Does he make colonialism look like a good or a bad thing?
What is the narrator's perspective in the text? The text is obviously narrated in third person, but what can we tell about the narrator? What side is he on? What opinion does he have of the events in the text? If you can figure this out, you'll figure out the narrator's perspective.