Possible Clues:

• Frequently, in old literature
• Frequent
• 'So ___ have I invoked thee ...': Shak.
• It's common in poetry
• Frequently, for Frost
• Often, to Standish O'Grady
• 'How ___ is the candle of the wicked put out!': Job 21:17
• It's frequently in verse
• T's the frequency of this
• Frequently (poetic)