Principal character in a line (8)
I believe the answer is:
cardinal
'principal' is the definition.
(thesaurus)
'character in a line' is the wordplay.
'character' becomes 'card' (I've seen this before**).
'line' becomes 'l'.
'card'+'in'+'a'+'l'='CARDINAL'
(Other definitions for cardinal that I've seen before include "Word to describe one or two" , "Vivid red of a senior bishop?" , "Main - church dignitary" , "of prime importance" , "papal adviser" .)