Possible Clues:

• Pen type
• Type of pen
• Felt-tip alternative
• 'Nowadays round the end it has taken over, all right, by the sound of it (4-5)'
• Writer of book everyone aims to follow
• Paper Mate product
• Great fun setting the pinto loose in the pen
• Having a great time training a pinto for the pen
• Monster revealed in tea-leaf ritual
• Kind of pen