Using a Marketo Webhook cannot work. A Marketo webhook is an outbound API that Marketo can call and that is always related to an existing lead.
What you want to do it to use Marketo Rest API or Marketo form API or the Save2 endpoint and call them from your third party solution. Among these, the Save2 endpoint can be called from a third party webhook, in other terms, a webhook fired from Clearbit.
Be aware that adding new leads to your database without their individual explicit and well informed consent it quite a risky approach and will be totally illegal in all the EU in 8 days. This is called GDPR
A webhook can loop back and use the Marketo REST API to manipulate leads other than the lead that executed the 'hook. Obviously this entails custom code on the webhook platform.
But this is hardly the best way to add leads to Marketo, as you will smack up against API limits. What is the exact use case for creating additional leads *in real-time* based on passing a lead to a Clearbit webhook (or to an intermediate hook to that processes Clearbit results) rather than in batch?