In conclusion, the setter is correct (5)
I believe the answer is:
emend
'is correct' is the definition.
(to emend something is to correct or improve it)
'in conclusion the setter' is the wordplay.
'in' means one lot of letters goes inside another.
'conclusion' becomes 'end' (something's conclusion is its end).
'the setter' becomes 'me' (the person who set this crossword).
'end' enclosing 'me' is 'EMEND'.
(Other definitions for emend that I've seen before include "get rid of faults" , "Make improvements to (text)" , "Edit, improve" , "Make improvements or corrections" , "Make correction in written material" .)