Leading light found ultimately lacking (4)
I believe the answer is:
star
'leading light' is the definition.
(I've seen this in another clue)
'found ultimately lacking' is the wordplay.
'found' becomes 'start' (as in to establish).
'ultimately lacking' means to remove the last letter.
'start' with its last letter removed is 'STAR'.
(Other definitions for star that I've seen before include "personality?" , "virtuoso" , "A .... called Henry (Roddy Doyle)" , "One guiding" , "Polaris, possibly" .)