Undisturbed when missing odd pieces for explosive component (5)

I believe the answer is:

nitre

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'explosive component' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for nitre that I've seen before include "Gunpowder ingredient" , "Mineral used in gunpowder" , "Inert (anag) - saltpetre" , "safeguard against decay" , "food preservative" .)

I've seen this clue in The Irish Times.

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