Lots of planes waiting to land (6)

I believe the answer is:

stacks

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'lots' is the definition.
('stack' can be a synonym of 'lot')

I don't understand how the remainder of the clue works.

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

I've seen this clue in The Globe and Mail.

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