Which action hook to use for function?

You don’t need an action, but a filter, because you want to do something with the result of a function. The filter is, predictably, called category_description In this way (didn’t test it though):

function wpse236947_trim_category_desc ( $desc, $cat_id ) {
    // do your thing
    return $desc;
}
add_filter( 'category_description', 'wpse236947_trim_category_desc' );