Opening sign (7)
I believe the answer is:
initial
I believe this is a double definition.
'opening' is the first definition.
(thesaurus)
'sign' is the second definition.
(initial is a kind of sign)
(Other definitions for initial that I've seen before include "Sign" , "Occurring at the beginnng" , "E in Eileen" , "write two or three letters, usually" , "Introductory" .)