Endorse a letter (7)
I believe the answer is:
This clue is a double definition.
'endorse' is the first definition.
(I've seen this before)
'a letter' is the second definition.
(initial is a kind of letter)
(Other definitions for initial that I've seen before include "Name's first letter", "Sign", "Let; rent", "Word's first letter", "Occurring at the start".)