First add options.
add_option( 'clusters', 'End date.');
register_setting( 'seqform_options_group', 'clusters', 'seqform_callback' );
Multiple select
<select name=”clusters[]” multiple=”multiple”>
<option value=’one’>One</option>
<option value=’two’>Two</option>
<option value=’three’>Three</option>
</select>
