The theme of the underdog? (7)
I believe the answer is:
subject
'the theme' is the definition.
('subject' can be a synonym of 'theme')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for subject that I've seen before include "In school possibly English" , "Theme (of a fugue)" , "issue" , "Expose; field of study" , "One is under the authority of royalty" .)