Harbour bar in which business is unusually brisk (4)

I believe the answer is:

boom

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'harbour bar' is the definition.
'boom' can be an answer for 'bar' (thesaurus). I am not certain of the 'harbour' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for boom that I've seen before include "Harbour barrier" , "time of 22 [PROSPERITY]" , "Period of prosperity - explosive sound - sailing boat's spar" , "Loud deep sound" , "Spar - loud noise" .)

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

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