One can reply to a comment using
Reply link, below each comment item in comments list.
According to codex, by clicking that link and using
comment_form_title function like this:
$replytext = 'Leave a Reply to %s'; // %s = author comment_form_title( $noreplytext, $replytext, $linktoparent );
You can have a comment form title as
Leave a Reply to [author's name].
I would like to find a way in which you have this output instead:
Leave a Reply to [comment's excerpt]
where comment's excerpt is the comment's first few words.