Map post excerpt field

web/modules/custom/custom/migrations/migrate_plus.migration.post_node.yml

@@ -22,6 +22,10 @@ source:
       selector: title
       label: Title
 
+    - name: excerpt
+      selector: excerpt
+      label: Excerpt
+
     - name: created
       selector: created
       label: Created
@@ -55,6 +59,8 @@ process:
   path: path
   status: status
 
+  field_excerpt: excerpt
+
   field_tags:
     plugin: migration_lookup
     migration: tag_term