Possible Clues:

• Second day of Christmas arrivals (with 38-Across)
• Michelangelo or Donatello, e.g
• Cold-blooded creature let off after brief spell
• Shelled creature
• This swimmer FBI's clobbered with rifle-butt?
• Time to race along — hard going for distance swimmer
• Felt ruthless, turning to block swimmer
• Possible aquarium dweller
• Marine reptile of the order Chelonia
• Reptile to go off, briefly let out