Leading article on top (5)
I believe the answer is:
ahead
'leading' is the definition.
(to lead is to go ahead)
'article on top' is the wordplay.
'article' becomes 'a' (English indefinite article).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'top' becomes 'head' (synonyms).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "Forward - in front" , "Forwards" , "at pole" , "Winning" , "In the vanguard" .)