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.
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.
Again, adding an expletive is possible but the sentence is still without a subject:
Es wird von ihm hart gearbeitet.