Fractions of this mixed with two lots of nitrogen (6)

I believe the answer is:

ninths

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'fractions' is the definition.
(I know this)

I cannot understand how the remainder of the clue works.

(Other definitions for ninths that I've seen before include "Intervals of just over an octave" , "There are 7 of these parts in 63, say" .)

I've seen this clue in The Times.

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