Crossword Genius

A number of fingers (5)

Ross

I believe the answer is:

frost

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This might be a double definition.

'a number' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'of fingers' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

(Other definitions for frost that I've seen before include "Rime" , "Frozen deposit" , "Icy deposit" , "Ice crystals forming a white deposit" , "Robert ---, US poet" .)

I've seen this clue in The Globe and Mail.
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