There's two ways to go about this that I can think of.
Option A
You can have separate Content Element code for the listing page vs the single-entry page:
Listing page:
{content_a}
{wygwam:excerpt}
{value}
{/wygwam}
<a href="#">Read More...</a>
{/content_a}
Single-entry page:
{content_a}
{wygwam} // shows the entirety of the Wygwam field
{value}
{/wygwam}
// Also include the other field types:
{image} ... {/image}
{video} ... {/video}
{/content_a}
Option B
You can have an entirely separate Wygwam field (called content_summary
here) for the summary/excerpt, leaving the Content Elements field dedicated to the full article content:
Listing page:
{content_summary}
<a href="#">Read More...</a>
Single-entry page:
{content_a}
{wygwam}
{value}
{/wygwam}
// Also include the other field types:
{image} ... {/image}
{video} ... {/video}
{/content_a}