Citizen forms the theme (7)
I believe the answer is:
subject
This clue is a double definition.
'citizen' is the first definition.
(I've seen this before)
'the theme' is the second definition.
(synonyms)
'forms' is the link.
(Other definitions for subject that I've seen before include "Musical theme" , "Put down" , "Theme, topic" , "One is under the authority of royalty" , "issue" .)