Make corrections to pieces penned by editor (5)
I believe the answer is:
Here is my best explanation:
'make corrections' is the definition.
(I've seen this before)
'pieces penned by editor' is the subsidiary indication.
'pieces' becomes 'men' ('man' can be a synonym of 'piece').
'penned by' means one lot of letters goes inside another.
'editor' becomes 'ed'.
'men' inserted within 'ed' is 'emend'.
'to' is the link.
(Other definitions for emend that I've seen before include "Edit; make alterations to a draft", "Improve by editing", "do editorial work", "Make correction in written material", "Correct/revise (a text)", "correct and revise", "Remove errors", "Remove faults from", "make corrections" and "Polish".)