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