Mark in the margin (6)
I believe the answer is:
notate
'mark in the margin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for notate that I've seen before include "Write using signs or symbols as in music etc" , "Use symbols to record (eg, music)" , "Put down in symbols" , "Record, say" , "Express in symbols" .)