Theme Republican found in the publicity (6)
I believe the answer is:
thread
'theme' is the definition.
(thesaurus)
'republican found in the publicity' is the wordplay.
'republican' becomes 'r' (abbreviation).
'found in' indicates putting letters inside.
'publicity' becomes 'ad' (advert).
'r' put within 'the' is 'thre'.
'thre'+'ad'='THREAD'
(Other definitions for thread that I've seen before include "Filament; connecting link" , "Prominent spiral part of a screw" , "Cord - Internet chat room discussion" , "Sewing chord" , "clothes casually cut" .)