Hub of activity (4)
I believe the answer is:
hive
'hub of activity' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hive that I've seen before include "Bee colony" , "Where to get honey" , "Insects' home - site of much activity" , "where people are busy" , "Place swarming with activity" .)