When you clicked the link.

1. It calls addP.
2. It removes onclick handler.

Now the actual HTML looks like

<a href="#" onclick="addP();">See more</a> 

To fix this remove onclick='' part in the onclick attribute.

let addP = () => alert('you clicked the link')

<a href="#" onclick="addP();">See more</a>    

You have to remove this.onclick=''; because when the o'clock event (addP) Is over, it removes the the click event handler. So there are 2 methods for you :

Method 1
use javascript & html like so :

HTML :

<a href="#" id="seeMore">See more</a>    

Javascript :

document.getElementById('seeMore').addEventListener('click', function() {
    addP();
});

Method 2
only using html

<a href="#" onclick="addP();">See more</a>