The immediate, facile answer is that "yes, it's fine, since so many people have done it already"... and, also, since your audience will certainly understand the content.
A secondary point is that quite often a more effective-and-efficient sentence structure is available, as exampled in other answers and comments. In some ways, these secondary things don't truly matter, but they can distract the reader, as well as allowing a critical reader to wonder about what you think is happening... which is probably not good.
An exaggerated case is "Let the real numbers be complete." What? Well, yes, they are, but what is the point of saying this? Or "Assume $x$ is a real number." Wait, what? What if it's not? Does one mean "here let $x$ be a real number..."? Probably. And, yes, experienced readers can decrypt the weird syntax, but there's no point in making them do so.
Alternatives to "Let $x$ be a real number..." are, for example, "For real numbers $x$, ..." and such.