Duty of a queen intervening in row (6)
I believe the answer is:
tariff
'duty' is the definition.
(synonyms)
'a queen intervening in row' is the wordplay.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'intervening in' means one lot of letters goes inside another.
'row' becomes 'tiff' (tiff is a kind of row).
'a'+'r'='ar'
'ar' put inside 'tiff' is 'TARIFF'.
'of' is the link.
(Other definitions for tariff that I've seen before include "List of fixed charges as in hotel" , "Import/export duty" , "Sentencing scale" , "Price list - import tax" , "Table of fixed charges" .)