Busy airport (3)

I believe the answer is:

'busy airport' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for hub that I've seen before include "Centre (of network)" , "Focus" , "Centre of wheel" , "Centre or axis" , "Central part of a car wheel" .)

I've seen this clue in The New York Times, the USA Today, The Mirror, the King Feature Syndicate and The Wall Street Journal.

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