ChEMBL targets are primarily associated with Uniprot accessions so you'll probably need to do it in two steps. The first is to retrieve the Uniprot accession number, the second to convert Uniprot accession to Entrez gene ID. For the first step, you have a few options:

• You can retrieve Uniprot accessions mapped to a ChEMBL target ID from Uniprot's Retrieve/ID mapping service by selecting ChEMBL as "From" option.

• There's also the ChEMBL REST API, e.g.
  https://www.ebi.ac.uk/chembl/api/data/target/search?q=CHEMBL6196
  This gives access to other IDs than Uniprot accession, e.g. Ensembl gene IDs or gene symbol which may be easier to convert to Entrez gene IDs. Default format is XML but you can get json or yaml by adding e.g. &format=yaml to the request.

• There's also the option of downloading the database and performing SQL queries on it (with some examples here). This is useful if you're going to do this a lot and remote queries via the web are taking too long.