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Extremely
small
fraction
of
hour
(6)
Extremely small fraction of hour (6)
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The whole clue is the definition (& lit or double definition).
How the clue works
Look at Extremely small — that's one definition, giving you the adjective meaning tiny (pronounced my-NOOT). Then fraction of hour does the same job from the other direction, since sixty seconds is exactly that. Both meanings share the same spelling, which is what makes this a lovely double definition.