You can use "must" in guesses, predictions, and explanations:
A: Where's your phone?
B: Oh, I don't know. It must be in my jacket pocket or something.
In the example at top, the man thinks "It must be clogged." "Clogged" is an adjective. Here are some other examples of "___ must be" followed by an adjective:
It must be expensive!
Your mother must be so proud of you!