Possible Clues:

• Flounder kin
• Crude shelter in which a politician's stored fish
• Henry I could have started it, although that's more than a little fishy
• Large flatfish, tasty to eat
• Henry the First may have started it, although that's flat to a large extent
• Henry the First got 27 across back, and that's flat
• Large flatfish used as food
• 9 down in short, although that makes it largely fishy
• Henry I's only fish
• Fishy sort a politician's hiding in shelter