First sanction it in writing (7)
I believe the answer is:
initial
I believe this clue is a double definition.
'first' is the first definition.
(similar in meaning)
'sanction it in writing' is the second definition.
(to approve something by signing your initials)
(Other definitions for initial that I've seen before include "Endorse" , "subscribe, minimally" , "First letter (of a word)" , "First letter of a name" , "Introductory" .)