First mark made on page of official document (7)
I believe the answer is:
initial
'first' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for initial that I've seen before include "write two or three letters, usually" , "Start of name" , "Primary" , "Sign" , "Occurring at the beginnng" .)