Partial Index Reset of a single content source
This script will remove and re-add your content source‘s start addresses.
SharePoint will more or less rebuild the index for these sources, when the next full crawl is started.
$sourceName = "Local SharePoint sites"
$SSA = Get-SPEnterpriseSearchServiceApplication
$source = Get-SPEnterpriseSearchCrawlContentSource -Identity $sourceName -SearchApplication $SSA
$startaddresses = $source.StartAddresses | ForEach-Object { $_.OriginalString }
$source.StartAddresses.Clear()
ForEach ($address in $startaddresses ){ $source.StartAddresses.Add($address) }