The underdog is the topic (7)
I believe the answer is:
subject
'the topic' is the definition.
('subject' can be a synonym of 'topic')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for subject that I've seen before include "Expose; field of study" , "Citizenship' at school maybe" , "guinea pig" , "Theme - one under rule" , "Branch of knowledge" .)