Worry wort's final sign of omission (5)
I believe the answer is:
caret
'sign of omission' is the definition.
(I've seen this in another clue)
'worry wort's final' is the wordplay.
'worry' becomes 'care' (eg without a care means without a worry).
'final' indicates one should take the final letters.
The final letter of 'wort' is 't'.
'care'+'t'='CARET'
(Other definitions for caret that I've seen before include "Weeding tool" , "this reveals something missing" , "Sign something's lacking" , "Proofreading sign for 'Insert'" , "It shows what to put in" .)