Charge harbour where there are low duties (3,5)
I believe the answer is:
tax haven
'where there are low duties' is the definition.
'charge harbour' is the wordplay.
'charge' becomes 'tax' (tax is a kind of charge).
'harbour' becomes 'haven' (synonyms).
'tax'+'haven'='TAX HAVEN'
(Other definitions for tax haven that I've seen before include "Where you'll save money" , "place for dodgy financiers?" , "Place where government financial demands are low" , "Could be Jersey" , "Man's one" .)