。
Most bricks are made of clay.
Literal
Bricks [of-の] most-part [topic-は] clay [from-から] consists.
からなる is an important set pattern: 'to be composed of, to consist of (material/parts).' Literally 'to become from,' but the meaning is stative — the thing is made out of the material. Takes から rather than で, which would suggest active construction. 大部分 ('large part, majority') here functions as the topic.