Beginning a letter (7)
I believe the answer is:
initial
This clue is a double definition.
'beginning' is the first definition.
(I know that initial is a type of beginning)
'a letter' is the second definition.
(initial is a kind of letter)
(Other definitions for initial that I've seen before include "Occurring at the start" , "E in Eileen" , "Occurring at the beginning" , "Occurring at the beginnng" , "Word's first letter" .)