frost is the answer. It's one of a number of numb-ers of fingers as you suggest. Another clue I have come across in a similar vein is "A flower of Paris (5)" to

frost is the answer. It's one of a number of numb-ers of fingers as you suggest. Another clue I have come across in a similar vein is "A flower of Paris (5)" to which the answer is Seine. These clues may or may not be to your liking. I like them.