Answer: subordinate clause: what one thinks; function: noun.
Explanation: a subordinate clause is a phrase that has a subject and a predicate but it doesn't have complete meaning on its own, it depends on an independent clause (a phrase that has complete meaning by itself) to make sense. In the given sentence, the subordinate clause is "what one thinks" it is acting like a noun (it is a noun phrase), specifically it is the subject of the sentence.
