Have influence giving support to national topic (7,6)
I believe the answer is:
subject matter
'topic' is the definition.
(I've seen this before)
'have influence giving support to national' is the wordplay.
'have influence' becomes 'matter' (matter can mean to have influence or be relevant).
'giving support to' means one lot of letters go next to another.
'national' becomes 'subject' (synonyms).
'matter' put after 'subject' is 'SUBJECT-MATTER'.
(Other definitions for subject matter that I've seen before include "Theme" , "Material at hand" , "citizen feel important" , "Book content" , "The topic being dealt with" .)