Assuming
a_CN_Affected_Item is a relationship, then the following would be the simplest way to accomplish what you want.
<Item type="a_CN" action="get" select="id,cn_number,title,a_businessunit,customer(keyed_name),top_assy_pn(keyed_name),team_id,change_reason,change_summary">
<id>${Item/id}</id>
<team_id>
<Item type="Team" action="get">
<Relationships>
<Item type="Team Identity" action="get">
<team_role>52C5CF6C021A443D845CF6F623EF48C4</team_role>
<related_id>
<Item type="Identity" action="get" select="name"></Item>
</related_id>
</Item>
</Relationships>
</Item>
</team_id>
<Relationships>
<Item type="a_CN_Affected_Item" action="get" select="affected_name">
</Item>
</Relationship>
</Item>
As a separate query, both of the following should also work
<Item type="a_CN_Affected_Item" action="get" where="[a_CN_Affected_Item].source_id=${Item/id}" select="affected_name">
</Item>
OR
<Item type="a_CN_Affected_Item" action="get" select="affected_name">
<source_id>${Item/id}</source_id>
</Item>