The amount of applicable lines involving the scratched runner will be refunded, while the remainder of the bet will still stand.
For example, in the bet below, if number #3 was scratched, the applicable combos involving the scratched runner would be voided. In this case, as we have only selected 4 runners in total, a scratching would void the whole bet as it would not be possible for any combination to win without 4 selections running, so the full $10 stake would be refunded.
As another example, in the bet below, if #3 was scratched, the applicable lines/combos involving the runner will be voided/returned. In this case, as #3 is involved in 2 combos, $3.33 will be refunded and the remaining bet will stand.