SOLUCIONADO. Encontre uno mejor en el codex:
Código:
add_action('pre_get_posts', 'gloss_remove_glossary_cat' );
function gloss_remove_glossary_cat( $notused ){
global $wp_query;
global $gloss_category;
// Figure out if we need to exclude glossary - exclude from
// archives (except category archives), feeds, and home page
if( is_home() || is_feed() || is_search() ||( is_archive() && !is_category() )) {
$wp_query->query_vars['cat'] = '-5' . $gloss_category;
}
}