Harbour bar in which business is unusually brisk (4)
I believe the answer is:
boom
'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" .)