“It's supposed to clear up by Friday.”

It's been raining for a few days. You're making small talk with a coworker. She says that she hopes it doesn't rain on the weekend because she has plans to go out with a relative who is visiting. You heard from the weather report that the rain will end on Thursday or Friday. You say:

It's supposed to clear up by Friday.

it (is) clear

Use "it" to talk about the weather (or the temperature of a room):

It's a sunny, clear day.

It's a bit chilly, isn't it?

(something) is supposed to (do something)

"Supposed to" can be used to explain a strong prediction that someone has made and you have heard. A weather forecast is a perfect example. The newscaster has predicted what the weather will be like. You have heard this prediction, so you repeat it to another person as "it's supposed to ___".  Another example is a rumor or announcement that you hear at work:

We're supposed to be getting new laptops in a couple months.

"Supposed to" also has other meanings in other situations, like when you're telling someone what they should be doing:

You're supposed to call if you're going to be late.

clear up

This phrase means to become completely clear. It can be used to talk about weather, as in the example above. It can also be used to discuss an illness:

I hope your cold clears up soon.