Possible Clues:

• "What have we here?" and 20-, 36-, and 42-Across
• '. . . look what we have here!'
• “Just look what we have here...,” and a summary of 20-, 28- and 46-Across
• Start of a quip
• Echoic remark before "What do we have here?" whose words can follow the ends of 20-, 28- and 42-Across
• "Just lookee here"
• My thoroughly healthy mineral source