OK, first of all there are a couple of excellent tutorials that you should read before trying this (if you haven't done so already):
integrating leaderboards... by austin
how to make custom leaderboard by CanGame
It is also worth bookmarking the clay.io manual page for construct 2 here. Austin has done an outstanding job at not only creating the plugin but also documenting how to use it.
Now for the direct answer that you are after:
After posting the high score to clay.io, create an event to retrieve the leaderboard object, which will then be remembered by the plugin. You could easily show the top 10 scores this way as shown in the custom leaderboard tutorial above. The top score is contained at index 0 of the leaderboard - your event should look like this:
You also have the option to specify downloading the Personal leaderboard for the player, so you could choose to give them some banter as to why they haven't beaten their previous high score...