Crowded square increased in temperature (7)

I believe the answer is:

swarmed

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'crowded' is the definition.
(swarming is a kind of crowding)

I cannot understand how the rest of the clue works.

(Other definitions for swarmed that I've seen before include "Teemed (like bees)" , "crowded" , "(Of bees) flew en masse" .)

I've seen this clue in The Independent.

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