Possible Clues:

• First letters
• FDR and JFK
• They come first by letter, but in it at first
• Autographs the first letters
• Approves, in a way
• At first in it, sail. At first, letters
• They all come first - in it at first
• They're something to do with 10 across
• They're in it at the beginning
• They all come at first but are in it at first