Piles them up in library storage area (6)

I believe the answer is:

stacks

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'piles' is the definition.
(pile can mean a stack of something)

I cannot understand how the rest of the clue works.

(Other definitions for stacks that I've seen before include "Orderly heaps as of hay" , "bags?" , "A lot (of hay?)" , "Orderly piles or heaps" , "Large tall chimneys" .)

I've seen this clue in The Irish Times.

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