In my setting:
Druids draw their power from nature itself, not from a God. Specifically, their power originates in the positive and negative energy planes and flows through the Feywild and Shadowfell to reach the druid. They can worship a God just as a fighter might, but the Gods grant them nothing. Their organizations are more interested in the balance of the universe than the interests of the Gods. While they may be beholden to mortals in their organization, they are not beholden to extraplanar powers.
A cleric of nature is a lot like a cleric of magic. They represent the interests of a God who holds sway over a fundamental force of the universe. If they anger that God, the God may take their powers from them. They have a responsibility to others that worship that God. They have a duty to spread the word of the God and recruit more to the cause. They give their faith to the God, and the God grants them power. They are empowered at the whim of the Gods - and that relationship, just as any relationship, can be stable and supportive or abusive depending upon your partner. This approach holds true for a few non-cleric subclasses as well, such as order of the ancients paladins, zealot barbarians, and certain monks.
A third related category are warlocks affiliated with life, death or fey (Archfey, Hexblade, Undead, Undying). They have contracts by which they are granted power over the fundamental forces tied to nature and the lifecycle. Often, those that care most about nature are tempted to make deals with these beings that pervert or twist nature to their bidding. The warlocks have to follow the terms of their contract with the greater being that gave them their power, but beyond that they are free agents which allows them to protect nature with tools tied to the life and death struggle of nature. Sometimes those contract terms are very specific and highly monitored, and at other times they are very vague and there can be disputes about whether the warlock is in conflict - but either way, the warlock usually learns that their contract is very, very important. The warlocks with an interest in the natural balance often find themselves at odds with the power that granted them their abilities as Archfey, Shadow, and Undead forces tend to be abusive of the forces of nature in one way or another.