Best How To :

If I understood well you are asking for ContentElement preview. You need to use cms/layout/class.tx_cms_layout.php hook for this, here's quite nice gist

just two additions:

1. don't use t3lib_extMgm class it's removed since 7.x you can register this hook just with:

   $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['cms/layout/class.tx_cms_layout.php']['tt_content_drawItem'][$_EXTKEY] 
   =  'EXT:your_ext/Classes/Hooks/PageLayoutView.php:\Vendor\YourExt\Hooks\PageLayoutView';
   

2. Depending on how did you register the plugin (didn't mention) you can also need to check the $row['list_type'] as your $row['CType'] may be just generic list.

Sample class with value from FlexForm field

<?php

namespace Vendor\YourExt\Hooks;

class PageLayoutView implements \TYPO3\CMS\Backend\View\PageLayoutViewDrawItemHookInterface {

    public function preProcess(\TYPO3\CMS\Backend\View\PageLayoutView &$parentObject, &$drawItem, &$headerContent, &$itemContent, array &$row) {
        if ($row['CType'] == 'list' && $row['list_type'] == 'yourext_yourplugin') {

            $drawItem = false;

            $linkStart = '<a href="#" onclick="window.location.href=\'../../../alt_doc.php?returnUrl=%2Ftypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D' . $row['pid'] . '&amp;edit[tt_content][' . $row['uid'] . ']=edit\'; return false;" title="Edit">';
            $linkEnd = '</a>';


            $headerContent = 
                $linkStart . 
                "<strong>Selected slides</strong>" .
                $linkEnd;


            $ffXml = \TYPO3\CMS\Core\Utility\GeneralUtility::xml2array($row['pi_flexform']);

            $itemContent =
                $linkStart .
                $ffXml['data']['sDEF']['lDEF']['settings.myFlexField']['vDEF'] .
                $linkEnd;
        }
    }
}