Line on a Harrod's bill (5-5,3)
I believe the answer is:
value-added tax
'line on a harrod's bill' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for value-added tax that I've seen before include "Duty on goods and services" , "Levy on most goods and services" , "Adult Dave axed (anag) - impost, currently set at 17.5%" , "VAT" , "Levy on the difference between cost of materials and selling price of commodities" .)