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