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Central
parts
of
genome
system
purify
cells
forming
air
passage
(7)
Central parts of genome system purify cells forming air passage (7)
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The definition is at the end of the clue.
How the clue works
The definition is air passage, which a nostril is. Taking the central parts of consecutive words gives NO (geNOme) + ST (sySTem) + RI (puRIfy) + L (ceLls), and 'forming' assembles them into NOSTRIL.