custom_override_checkout_fields.php

<?php

// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
  $fields['billing']['billing_company']['label'] = 'Clinic Name';
  $fields['shipping']['shipping_company']['label'] = 'Clinic Name';
  $fields['billing']['billing_first_name']['label'] = 'Physician First Name';
  $fields['billing']['billing_last_name']['label'] = 'Physician Last Name';
  $fields['shipping']['shipping_first_name']['label'] = 'Physician First Name';
  $fields['shipping']['shipping_last_name']['label'] = 'Physician Last Name';
  $fields['billing']['dea_number'] = array(
    'label'     => __('DEA Number', 'woocommerce'),
    'placeholder'   => _x('If you are already registered, make sure to use your approved DEA Number', 'placeholder', 'woocommerce'),
    'required'  => true,
    'class'     => array('form-row-wide'),
    'clear'     => true
  );
  return $fields;
}

/**
 * Display field value on the order edition page
 **/
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
function my_custom_checkout_field_display_admin_order_meta($order){
  echo __('DEA Number'). ': ' . $order->order_custom_fields['_dea_number'][0];
}

custom_override_checkout_fields_2-1.php

<?php

/**
 * Display field value on the order edition page
 **/
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
function my_custom_checkout_field_display_admin_order_meta($order){
  echo __('DEA Number').': ' . $order->dea_number;
}