Partisan as certain as could be (9)
I believe the answer is:
sectarian
'partisan' is the definition.
(sectarian is a kind of partisan)
'certain as could be' is the wordplay.
'could be' is an anagram indicator.
'certain'+'as'='certainas'
'certainas' anagrammed gives 'SECTARIAN'.
'as' is the link.
(Other definitions for sectarian that I've seen before include "from an unorthodox group" , "Concerning one group" , "Ascertain to be with religious prejudice" , "Prejudiced against other religions? So ascertain" , "Denominational" .)