Possible Clues:

• Something to worry about
• Activist's concern on paper about gangster meeting a Russian leader - it's reason to be fearful
• Reason for concern; potential scare
• In the Line of Fire, for instance, one needs to rouse oneself for this
• Novel by Eric Ambler
• Having to get up early? That's frightfully!
• Sounds as if there must be some frightful reason for such frightful sounds in the rookery
• By the sound of it, the sort of frightful noise that would come from the rookery
• The frightful sound a rookery makes? They have some reason for it