I wanted [to see a lion]! Wanted what? The bracketed infinitive clause acts as the direct object of "wanted".
Why "to see a lion" is a clause? Each clause has a subject. This one has too. It is just that you can't see it. The verb in this clause is "to see", a verb in infinitive form (see) preceded by a preposition "to", an infinitive marker.
Wherever Melissa goes, she always brings a book to read.
to read = infinitive in form; adjectival postmodifier in function; it modifies "book"
You have more questions, whistle!
Student or Learner