Reading the description of the Light domain cleric's Channel Divinity: Radiance of the Dawn, I stumbled over the words "harness sunlight":
Starting at 2nd level, you can use you Channel Divinity to harness sunlight, banishing darkness and dealing radiant damage to your foes. (PHB, p. 61, emphasis mine)
Does that mean you have to be outside (or at least have a partial view of the sky) to use that feature?
Best Answer
No
Strict limitations like that are told to you in the ability's description. Since the ability does not say it must be used outside, it doesn't have to be.
Additionally, there is substantial evidence that the designers already had experience with requiring or taking into account environmental conditions when designing spells and effects. The Call Lightning spell allows you to take control of an existing storm if you are outside in stormy weather. Additionally, the Control Weather spell requires you to be outside and maintain a clear path to the sky.
Since the limit for being in a clear view of the sky is already used in the Control Weather spell, and since the Call Lightning spell has a special condition when you are outside, we know for a fact that the designers were aware of the effect of the environment when casting spells. Since Radiance of the Dawn does not list any requirements for being outside, and because these limitations exist in other places, it is reasonable to assume that the designers would have listed a requirement on Radiance of the Dawn if one were intended. No such requirement is listed, and thus we can reasonably assume that no limit in terms of being outside was intended.
As a thematic parallel, there are two spells in the PHB, Sunburst and Sunbeam that can actually create sunlight, neither of which requires you to be outside. Since we know that sunlight can be magically generated, it isn't beyond belief that a Light Domain Cleric would have the power to harness sunlight at will.