This guide will tell you where to get the Clashing Forest in Warframe. You have to follow the simple steps in this guide to get it.
The clashing forest can drop from corrupted ancients which you can find in void fissure missions and missions in the void. You can run Ani in the void because the ancients will spawn throughout the rest of the mission.
After that, they just continued to spawn throughout the mission, so you shouldn’t have to be in the mission for very long before ancients begin to spawn.
This mod has a fairly low drop chance, so when you start farming for the mod, you have to prepare yourself as you’re probably going to be doing so for some time unless you get lucky on your first few runs.
When farming for mods that drop from enemies, bringing Nekros is more or less always a good idea since he can make enemies drop extra loot/items.