Setter, in conclusion, is correct (5)
I believe the answer is:
emend
'is correct' is the definition.
(I've seen this before)
'setter in conclusion' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'in' means one lot of letters goes inside another.
'conclusion' becomes 'end' (something's conclusion is its end).
'me' put inside 'end' is 'EMEND'.
(Other definitions for emend that I've seen before include "Improve (texts)" , "would appear better" , "change" , "Alter to improve (text)" , "get rid of faults" .)