ORDER BY custom field value

Based on example here https://make.wordpress.org/core/2015/03/30/query-improvements-in-wp-4-2-orderby-and-meta_query/

I’d like to modify the query

$q = new WP_Query( array(
'meta_query' => array(
    'relation' => 'AND',
    'state_clause' => array(
        'key' => 'state',
        'value' => 'Wisconsin',
    ),
    'city_clause' => array(
        'key' => 'city',
        'compare' => 'EXISTS',
    ), 
),
'orderby' => 'city_clause') );

to be able to get all posts where state is ‘Wisconsin’ OR timezone is ‘central’ ORDER BY population DESC

You can create groups of meta_queries using specific compare operation on them, and since you want to order based in a single custom field, you can keep the order declaration dedicated to the single meta field. So:

$q = new WP_Query( 
    array(
        'meta_key' => 'population', //setting the meta_key which will be used to order
        'orderby' => 'meta_value', //if the meta_key (population) is numeric use meta_value_num instead
        'order' => 'DESC', //setting order direction
        'meta_query' => array(
            'relation' => 'AND', //setting relation between queries group
            array(
                'relation' => 'OR', //setting relation between this inside query
                array(
                    'key' => 'state',
                    'value' => 'Wisconsin',
                ),
                array(
                    'key' => 'timezone',
                    'value' => 'central',
                )
            ),
            array(
                'key' => 'city',
                'compare' => 'EXISTS',
            )
        )
    )           
);