Possible Clues:

• He's not always a beast
• Boatman majestically on rising river: it's affected by the full moon
• Mythical monster — or we flew (anag)
• One likely to howl at a full moon
• Best bridge pair upset one who can become beastly
• 6 down sees the Guardian taking stock, perhaps, in retreat
• Wife was blooming nearly getting over man going wild occasionally
• Periodic monster
• Monster
• Professor Lupin in Harry Potter books, e.g.