In Discourse, a “topic” means all the messages. The first post is the original post (OP) or first post.
I understand however that it can be confusing since the button 
create a topic consisting on 1 post at first, and more posts when there are replies.
That said,
Would mean “only the first post of a topic” and it is indeed available, it’s the filter in:first:
Searching in all posts except the first ones isn’t possible, and at this time there is no operator to exclude a term, a filter, a category or anything else.