General antonyms:
* Open space: This is a broad antonym that refers to an area without dense vegetation.
* Clearance: This implies that the undergrowth has been removed.
* Open ground: This is similar to "open space" but specifically refers to the ground itself.
More specific antonyms:
* Canopy: If you're talking about the lack of undergrowth due to a dense canopy of trees, "canopy" is a good antonym.
* Prairie: If you're talking about a lack of undergrowth due to a flat, grassy area, "prairie" might be appropriate.
* Forest floor: This is the opposite of undergrowth in the sense that it refers to the bare ground beneath a forest.
The best antonym will depend on the specific context and the intended meaning.