$data=
[
    [
        "contract_id"=>"2",
        "number"=> "444",
        "start_date"=> "2020-10-10",
        "direction"=> "true",
        "state_id"=> "2",
        "sum"=> "22,00 ₽",
        "is_active"=> "true",
        "contra"=> [
            
                "contra_id"=> "1",
                "contra_name"=> "hgjgh"
            
        ]
    ],
    [
        "contract_id"=> "3",
        "number"=> "fg",
        "start_date"=> "2020-11-11",
        "direction"=> "false",
        "state_id"=> "2",
        "sum"=> "33,00 ₽",
        "is_active"=> "true",
        "contra"=> [
            
                "contra_id"=> "2",
                "contra_name"=> "Купи-продай"
            
        ]
    ]
];

 foreach($data as $key=>$arr){
                       $values[]=$arr['contra'][0]['contra_name'];
                      
                    }
                    array_multisort($values, SORT_ASC, $data);