Kind of rock from a pair of organs, mostly (5)

I believe the answer is:

magma

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'kind of rock' is the definition.

I can't explain the remainder of the clue.

(Other definitions for magma that I've seen before include "Hot fluid rock" , "hot stuff" , "Glassy volcanic rock" , "Molten or pasty rock" , "hot filling in crust" .)

I've seen this clue in The Times.

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