The labeling on the Cambridge dictionary website is slightly confusing. Looking at the definition for worsening, we see it is labeled "U or S".
- "U" means "Uncountable or singular noun: a noun that has no plural." I think it's saying that if a noun is uncountable, it must be singular. This is true. There is no such thing as a plural uncountable.
- WRONG: "Let's listen to some musics."
- RIGHT: "Let's listen to some music."
- WRONG: "I ate a bowl of rices."
- RIGHT: "I ate a bowl of rice."
- "S" means "A singular noun." There are no additional details.
In other words, "U or S" means that worsening can be used as an uncountable (and therefore singular) noun, or as a "plain" singular noun.
In your example sentence, worsening is not an uncountable noun. The author is referring to a single, particular instance of the economy getting worse. Its count is one. This is why it is correct to use an indefinite article. You could also use a definite article. ("The dramatic worsening of the economy caught everyone by surprise.")
More generally, many nouns can shift back and forth between being uncountable and being countable, depending on the context. For example, I could speak of "the rices of China" if I was talking about different breeds or varieties of rice.
For more detail, see some of these other questions: