foo.yaml

---
- name: foo
  hosts: localhost
  vars:
    bar:
      c:
        b:
          a: 1
  tasks:
    - name: bite me
      set_fact:
        deployment:
          clusters:
            - name: foo
              value: 4
            - name: bar
              value: 6
            - name: baz
              value: 8

    - name: make a list of clusters with the additional fact
      set_fact:
        temp: "{{ temp + [ item | combine(fubar, recursive=true) ] }}"
      vars:
        temp: []
        fubar:
          not_random_prefix: "12345"
      with_items: "{{ deployment.clusters }}"

    - name: Wassssuuuuuppppp
      debug:
          var: temp
  
    - name: replace the old clusters list with the new one
      set_fact:
        deployment: "{{ deployment | combine(dict, recursive=true) }}"
      vars:
        dict:
          clusters: "{{ temp }}"
  
  
    - name: Wassssuuuuuppppp
      debug:
          var: deployment