We hypothesize that conversational implicatures are a rich source of clarification requests, and in this paper we do two things. First, we motivate the hypothesis in theoretical, practical and empirical terms and formulate it as a concrete Clarification Potential Principle: implicatures may become explicit as fourth-level clarification requests. Second, we present a framework for generating the clarification potential of an instruction by inferring its conversational implicatures with respect to a particular context. We evaluate the framework and illustrate its performance using a human-human corpus of situated conversations. Much of the inference required can be handled using classical planning, though as we shall note, other forms of means-ends analysis are also required. Our framework leads us to view discourse structure as emerging via opportunistic responses to task structure.