“That's a real letdown.”
A baseball player on your favorite team was caught using drugs to improve his performance. You just saw the news about it on TV. You're disappointed. You think this to yourself.
That's a real letdown.
You use "very" and "really" with adjectives like this:
That's very dangerous. You shouldn't be doing that.
He's a really nice guy.
But you can't use "very" to modify nouns. Instead, you can use "a real ___":
When you had high expectations for something, but it disappointed you, you can say "That's a letdown."
A "letdown" is something that's disappointing. for example:
The phrase "That's a letdown" is pretty specific, though. You say it when something has just disappointed you, and you're feeling a little sad.
For example, you might say this if you read in the newspaper that your favorite bookstore is shutting down, or if your daughter tells you that she didn't get into the college that she applied for.