How to get this value inside RETURN

by

This is more of a PHP question then a WordPress one but the answer is simple, just use PHP’s output buffer by wrapping the first code snippet eg:

//create a function for the filters

function get_filters(){
    $terms = get_terms('YOUR-TAXONOMY');
    if ( $terms ) {
        ob_start();
        ?>
        <ul id="portfolioFilter">
            <li class="filter" data-filter="all">All</li>
            <?php foreach ( $terms as $term ): ?>
            <li class="filter" data-filter="<?php echo $term->slug; ?>"><?php echo esc_html($term->name); ?></li>
            <?php endforeach; ?>
        </ul>
        <?php
        return ob_get_clean();
    }
    return false;
}

//and then call that function in your second snippet:

return 
'<div class="listings clearfix">' 
. '<li class="filter" data-filter="'. $term->slug .'>'. esc_html($term->name) .'</li>'
. '<div class="inner-div preview-wrap">'
. '<div id="portfolio-wrap">'
. '<div id="portfolio_thumbs" class="columns-3">'
. '<ul id="grid" class="sortablePortfolio clearfix">'
. get_filters() 
. '</ul>'
. '</div>'
. '</div>'
. '</div>';