Possible Clues:

• Spectacle?
• That'll give one monocular vision
• Reader's assistant easily disheartened, say, by young lady
• Lens
• It helps one ogle German girl
• One says with glee that that's a single see-through
• I see I need only one like this
• See through this to keep a watch on your tumbler
• Lens for aiding defective vision
• For example, girl is after English youth leader's monocle