Edited: The
woocommerce_get_product_terms
is deprecated since Woocommerce version 3
Go with the following as @datafeedr wrote in his answer:
global $product;
$koostis = array_shift( wc_get_product_terms( $product->id, 'pa_koostis', array( 'fields' => 'names' ) ) );
or even more compact:
global $product;
$koostis = $product->get_attribute( 'pa_koostis' );
Original answer:
$result = array_shift(woocommerce_get_product_terms($product->id, 'pa_koostis', 'names'));