Is the collective word clump of shady trees correct?

"Clump" is a correct collective noun, but it usually refers to a small group of trees close together. "Shady" describes the trees, not the collection.

Here are some better options for describing a group of shady trees:

* Grove: This is a good option for a small, clustered group of trees.

* Thicket: This implies a dense, tangled grouping of trees.

* Stand: This can refer to a larger collection of trees, often of the same species.

* Copse: This is another word for a small, dense group of trees.

You could also use a descriptive phrase:

* A cluster of shady trees

* A collection of trees casting long shadows

* A group of leafy trees offering welcome shade

Ultimately, the best word or phrase will depend on the specific context and the size and type of trees you're describing.