Possible Clues:

• Poet and prophet holding numbers up
• Shakespeare, notably
• Poet studied primarily where info is online always
• Shall I compare thee to a summer's day with half a career as linesman?
• Part of issue clear always, poetically, for Shakespeare, say
• Boy clear and always poetic, becoming an imitator of Shakespeare?
• Enters one building responsible for 20 down
• Browsing the web, taken in by visionary poet
• 25 across loses the head with taunt - the type you'd normally associate with a linesman
• Boy, turning ten, always poetic as writer of verse