Possible Clues:

• Homestead fitted with one genuine siren
• Warning sound from gun, crossing a line
• That'll tell one to go to blazes
• Warning - a rare film damaged
• One may go off if there's 17 across coming
• Warning, one not imagined in growing area?
• Warner making a rare film
• Warner set enclosing concrete area
• Gangster carrying gun round, warning to get out
• Company describing genuine article, a warning device