Mail written copy as an afterthought (10)
I believe the answer is:
postscript
'an afterthought' is the definition.
(a PS in a letter)
'mail written copy' is the wordplay.
'mail' becomes 'post' (synonyms).
'written copy' becomes 'script'.
'post'+'script'='POSTSCRIPT'
'as' is the link.
(Other definitions for postscript that I've seen before include "Extra lines" , "part of letter" , "Addition to a letter" , "Last added words" , "belated comment" .)