Solr 4.2 - what is _version_field?

If you remove it, you must also remove the transaction logging from solrconfig.xml. See the link.

add the below field definition inside "field" tag in schema.xml

<field name="_version_" type="long" indexed="true" stored="true" multiValued="false"/>

The _version_ field is an internal field that is used by the partial update procedure, the update log process, and by SolrCloud. It is only used internally for those processes, and simply providing the _version_ field in your schema.xml should be sufficient.

If you'd like information as to exactly what is going on with _version_, you can visit this website to learn about the "optimistic concurrency" update process, which uses _version_.