Mark something of one's ignorance (4)
I believe the answer is:
sign
'mark something' is the definition.
(to sign something is to mark it)
'of one's ignorance' is the wordplay.
'of' says the answer is hidden in the clue.
'SIGN' can be found hidden inside 'ones ignorance'.
(Other definitions for sign that I've seen before include "Public notice; Leo, say" , "Public notice; gesture" , "Show" , "Omen or indication" , "Indication; symbol" .)