In conclusion this writer is correct (5)
I believe the answer is:
emend
'is correct' is the definition.
(thesaurus)
'in conclusion this writer' is the wordplay.
'in' indicates putting letters inside.
'conclusion' becomes 'end' (end is a kind of conclusion).
'this writer' becomes 'me' (the person who is writing these clues).
'end' going around 'me' is 'EMEND'.
(Other definitions for emend that I've seen before include "Remove errors" , "Remove faults from" , "Modify" , "Edit; make alterations to a draft" , "Improve (texts)" .)