The person writing leading article that is ungenerous soul (6)
I believe the answer is:
meanie
'ungenerous soul' is the definition.
(a mean person)
'the person writing leading article that is' is the wordplay.
'the person writing' becomes 'me' (the person writing the clues).
'leading' is a charade indicator (letters next to each other).
'article' becomes 'an' (indefinite article).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'me'+'an'+'ie'='MEANIE'
(Other definitions for meanie that I've seen before include "Stingy person (informal)" , "Ungenerous person" , "Nasty piece of work?" , "Unkind person (informal)" , "spiteful person" .)