[Guenni007]

i now did it this way:
i removed the class s and set an id s

<form action="<?php echo esc_url( home_url( '/' ) ); ?>" class="searchform clearfix" method="get">
<label class="assistive-text" for="s"><?php _e( 'Search', 'cleanretina' ); ?></label>
<input id="s" class="field" name="s" type="text" placeholder="<?php esc_attr_e( 'Search', 'cleanretina' ); ?>">
</form><!-- .searchform -->

and this will do the trick