Crossword Genius

They double the decimal fractions (6)

Ross

I believe the answer is:

fifths

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'fractions' is the definition.
(fifth is a kind of fraction)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for fifths that I've seen before include "Fractions" , "... of these for 60%" .)

I've seen this clue in the Evening Standard.
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