As noted earlier, I've just finished reading Mark Thompson's excellent book on the clarity of Scripture: A Clear and Present Word. Clearly (excuse the pun) this doctrine is timely and important - in a world where people claim that the Bible either can't be understood, or can be understood any way we choose, or has been misunderstood for centuries (and now we have it right!), or requires the church to understand it for us, a right and carefully thought out understanding of this topic is vital.
One thing which struck me on this reading was Luther's precise articulation of different types of clarity in Scripture. First, there is the external clarity of the Bible. "God has graciously chosen to express himself in the ordinary conventions of human language." (A Clear and Present Word, 148-9) This is what makes the comprehension and context questions of a Bible Study possible. We can translate it, read it, understand it. We can see structure in composition and rhetorical argument. It is clear because God has accommodated himself to human language (although of course its God's language to start off with!).
Secondly, however, there is as Luther describes, an internal clarity. That is, to understand the Bible as what it truly is, the revelation of God to us, which requires the Spirit of God in us. "Understanding in the true Christian sense, is more than making sense of the words on the page [...] Scripture remains God's word by which he addresses the human heart. God's clear word is made clear to believers by God." (A Clear and Present Word, 149). And this of course, is the glorious 'a ha!' moment in a Bible Study. When we hear God speaking to us through the text. When we are brought face to face with the glory of God in Christ, and respond in repentance and faith, and praise and proclamation.