Chief character in a line (8)
I believe the answer is:
cardinal
'chief' is the definition.
(thesaurus)
'character in a line' is the wordplay.
'character' becomes 'card' (I've seen this in other clues**).
'line' becomes 'l'.
'card'+'in'+'a'+'l'='CARDINAL'
(Other definitions for cardinal that I've seen before include "Senior Roman Catholic" , "Main point of compass - most important" , "Main - church dignitary" , "Counsellor to the Pope" , "Type of number or prelate" .)