Hello. You did not enter the text to which the question refers, which makes it difficult to accurately compose an answer. However, I will try to help you in the best possible way.
A context clue occurs when you don't know a word, or one of its meanings, but the entire text, or another word in the same sentence, presents you with a context that allows you to understand what the unknown word represents.
In the case of your question, you should look for a word in the sentence that is related to the word "mission" and that is contextualizing it. Esssa is the context clue.