> Ihn interessiert nur, **was mit Verbrechen zu tun hat.**

That dependent clause you have here is a so called *subject clause*. Yep. That clause is the subject.

You could add an expletive *das* but it's still the dependent clause that is the subject.

> Ihn interessiert nur das, **was mit Verbrechen zu tun hat.**

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FYI, unlike Enlish clauses, German clauses don't require a subject. You can see this best with impersonal passive voice:

> Er arbeitet hart.

This sentence has no accusative object, so the only way to build a passive voice from it is without a subject:

> Von ihm wird hart gearbeitet.