From CS Lewis's other writings on the nature of God specifically, and the numinous more generally, this phrase likely means to suggest your last idea, "full clarity of what we sense". It may be so far beyond us that we literally cannot know what it means, or what it is, despite having ourselves just experienced it.
Lewis often refers to the difficulty we have either comprehending or translating what is Real (God) from the 'natural' constraints of our worldly plane; indeed, much of his life's work in apologetics is trying to hash out the challenging bits of theology, which is meant to be practical, so that ordinary folks may better be able to benefit. The gulf is wide. I'm reminded of Mere Christianity's description of the fellow who has an experience of God in the desert, an exceptional overwhelming of the senses (one might say) that the man can't connect to abstracted theology. Likewise, Lewis himself struggles with his own analogies and often points out in many cases where he himself is frustrated with falling short of what he is 'really' trying to explain or express.
In this light, we are currently stuck in this plane but sometimes get a glimpse of the numinous or perhaps the barest glimmer of aspects of God (like the man in the desert) that we perceive through our earth-bound senses but is nonetheless so far beyond those senses that we cannot even articulate to ourselves what just happened. The transposition (as from the full experience of the ocean down to a map, i.e. scrap of blue-colored paper) is too extreme. Something far beyond us has been experienced within our own frame -- yet this frame is too limited to perceive it, quite, much less boil it down into language. Another example is Lewis's descriptions of that exalting stab of joy he felt in a few experiences -- rare, but leading him inexorably to Christ. His descriptions suggest it, and if you've felt it you know exactly what he means, but the sensation is literally and completely beyond description.
Additional commentary can be found at Into the Wardrobe