A banner's principal business? (8)
I believe the answer is:
headline
'a banner's' is the definition.
(thesaurus)
'principal business?' is the wordplay.
'principal' becomes 'head' ('head' can be a synonym of 'principal').
'business?' becomes 'line' (line is a kind of business**).
'head'+'line'='HEADLINE'
(Other definitions for headline that I've seen before include "star" , "Prominent text" , "It's prominent in paper" , "Article title" , "Newspaper title" .)