Answer:
The word cleave in this context refers to cutting something, usually in half. Cleave, oddly enough, can also mean to stick together, but that is not the relevant meaning in this case. Cleavage, therefore, is literally cutting. However it is usually used in a metaphorical sense, to refer to a woman's breasts, because the shape of the two breasts can be imagined to be the result of something that was cut in half (although, of course, that is not actually how breasts come about).