Possible Clues:

• Bishop covered in black stuff from pen
• Gather there's a bit of fire about the bishop
• A pen-holder?
• Business rules written by metal pen
• Livestock pen (US)
• The O.K., e.g.
• Pen Romeo found amongst colourful skeletons
• Enclosure puts Rex in the pink
• Defensive ring of wagons
• Enclosure into which hunted animals are driven