Sailor has one very strong duty (6)
I believe the answer is:
tariff
'duty' is the definition.
(I know that duty can be written as tariff)
'sailor has one very strong' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'has' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'very strong' becomes 'ff' (musical abbreviation for fortissimo, 'very strong').
'tar'+'i'+'ff'='TARIFF'
(Other definitions for tariff that I've seen before include "Rate" , "Menu" , "Customs duty" , "Import duty, tax" , "Duty charge" .)