The theme of the underdog? (7)

I believe the answer is:

subject

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'the theme' is the definition.
('subject' can be a synonym of 'theme')

I don't understand the remainder of the clue.

(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" .)

I've seen this clue in The Irish News.

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