Insertion mark (5)
I believe the answer is:
caret
'insertion mark' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for caret that I've seen before include "Sign something's lacking" , "Proofreader's sign of omission" , "Proofreading sign for 'Insert'" , "A sign something is missing" , "Indicator of omission" .)