Corrects a conclusion in writing (6)
I believe the answer is:
amends
'corrects' is the definition.
(amending is a kind of correcting)
'a conclusion in writing' is the wordplay.
'conclusion' becomes 'end' (something's conclusion is its end).
'in' means one lot of letters goes inside another.
'writing' becomes 'ms' (abbreviation for manuscript).
'end' placed inside 'ms' is 'mends'.
'a'+'mends'='AMENDS'
(Other definitions for amends that I've seen before include "Fixes" , "Changes for the better" , "Make them to compensate" , "Make these to atone for wrong done" , "Redress" .)