It's essentially the opposite of a double entendre, which is a phrase with two or more possible meanings, often one of them being humorous or suggestive.
Example:
* Double Entendre: "I'm going to the bank." (Meaning: 1. I'm going to the financial institution, or 2. I'm going to the edge of the river)
* Single Entendre: "The dog is sitting on the rug." (There's only one possible meaning, no hidden interpretations)
The term "single-entendre" is often used to playfully highlight the straightforwardness of a statement, especially in contrast to a more ambiguous or suggestive expression.