Possible Clues:

• Not obvious
• Ace up one's sleeve?
• Chameleon's "Wow!"?
• Really good and mad (one's having left)? Not in the view of dismissive type
• "Fantastic!" to the old-school crowd
• Proverbially, it leads to madness!
• Hidden optical faculty all used up?
• Submerged, perhaps? Marvellous!
• How an escapee tries to be
• 'What the eye doesn't see ...' - blind and crazy?