The undoubted leader? (8,7)
I believe the answer is:
definite article
'the' is the definition.
'undoubted leader?' is the wordplay.
'undoubted' becomes 'definite'.
'leader?' becomes 'article' (leader is a type of newspaper article).
'definite'+'article'='DEFINITE ARTICLE'
(Other definitions for definite article that I've seen before include "Maybe the" , "Black and white feature in paper" , "part of speech" , "In English, it's always the" .)