Afterthought (10)
I believe the answer is:
postscript
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">afterthought</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a PS in a letter)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for postscript that I've seen before include "belated comment" , "Addition to a letter" , "Last added words" , "part of letter" , "PS" .)