Fundamental clergyman (8)
I believe the answer is:
cardinal
I believe this clue is a double definition.
'fundamental' is the first definition.
(synonyms)
'clergyman' is the second definition.
(cardinal is a kind of clergyman)
(Other definitions for cardinal that I've seen before include "Word to describe one or two" , "of prime importance" , "Possibly one" , "papal adviser" , "Main - church dignitary" .)