Copied a letter to the Editor (4)
I believe the answer is:
aped
'copied' is the definition.
('ape' can be a synonym of 'copy')
'a letter to the editor' is the wordplay.
'letter' becomes 'p' (**).
'to the editor' becomes 'ed'.
'a'+'p'+'ed'='APED'
(Other definitions for aped that I've seen before include "was imitative" , "Imitated in every aspect" , "Imitated, mimicked" , "Imitated like primate" , "took off" .)