Crossword Genius

One cuts two thirds of time off (6)

Ross

I believe the answer is:

sickle

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

I can't explain the remainder of the clue.

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(Other definitions for sickle that I've seen before include "One stands for Communist" , "Short-handled reaping tool" , "Reaping hook" , "Reaping implement" , "field-worker being a trimmer" .)

I've seen this clue in The Independent.
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