The clustering contrib extension requires solr-clustering-*.jar under /dist and all JARs under contrib/clustering/lib. Configure the file path according to your system file path in solrconfig.xml:

<lib dir="${solr.install.dir:../../../..}/contrib/clustering/lib/" regex=".*.jar" />
 <lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-clustering-d.*.jar" />