Improve pieces penned by editor (5)
I believe the answer is:
emend
'improve' is the definition.
(emending is a kind of improving)
'pieces penned by editor' is the wordplay.
'pieces' becomes 'men' ('man' can be a synonym of 'piece').
'penned by' means one lot of letters goes inside another.
'editor' becomes 'ed'.
'men' put inside 'ed' is 'EMEND'.
(Other definitions for emend that I've seen before include "change" , "make an improvement" , "Edit, improve" , "do editorial work" , "Make correction in written material" .)