Endorse the leading character (7)
I believe the answer is:
initial
I believe this clue is a triple definition!
'endorse' is the first definition.
(I've seen this before)
'the leading' is the second definition.
(thesaurus)
'character' is the third definition.
(initial is a kind of character)
(Other definitions for initial that I've seen before include "Beginning - first letter" , "Letter - opening" , "Name's first letter" , "Let; rent" , "Primary" .)