Sign one letter (7)
I believe the answer is:
initial
I believe this clue is a double definition.
'sign' is the first definition.
(initial is a kind of sign)
'one letter' is the second definition.
(initial is a kind of letter of the alphabet)
(Other definitions for initial that I've seen before include "Sign" , "Relating to the beginning" , "Approve" , "First home, corgi, ultimately, wagging tail" , "Original; beginning" .)