Sign first (7)
I believe the answer is:
initial
This clue is a double definition.
'sign' is the first definition.
(initial can mean to sign with an initial)
'first' is the second definition.
(similar in meaning)
(Other definitions for initial that I've seen before include "Endorse" , "E in Eileen" , "Occurring at the start" , "write two or three letters, usually" , "First home, corgi, ultimately, wagging tail" .)