[{"id":1,"Name":"Susie","ownership":{"ownershipContextCode":"C1"},"clientIds":[{"clientClusterCode":"clientClusterCode_1","clientId":"11"}]},{"id":2,"Name":"John","ownership":{"ownershipContextCode":"C2"},"clientIds":[{"clientClusterCode":"clientClusterCode_2","clientId":"22"}]}]

db.collection.aggregate([{$addFields:{"ownership.ownershipClientCode":{"$arrayElemAt":["$clientIds.clientClusterCode",0]}}}])