A phrase is a group of words without a subject-verb component, used as a single part of speech. It's different than a clause because clauses are groups of words containing a subject and verb.
In this case, the group of words is a clause:
"The championdidn't expect the challenger to be such a formidable opponent, but despite his small size, the challenger possessed brute strength."
Twain's The adventure of Huckleberry Finn is the excerpt which is about loyalty and friendship between two persons. The subject of the excerpt helps the readers to identify the theme. The subject helps audience to understand the theme of friendship and loyalty.
