The first to endorse (7)
I believe the answer is:
initial
I believe this clue is a double definition.
'the first' is the first definition.
(I've seen this before)
'to endorse' is the second definition.
(I've seen this before)
(Other definitions for initial that I've seen before include "Let; rent" , "Primary" , "Introductory" , "Endorse" , "Occurring at the start" .)