Service tax (4)
I believe the answer is:
duty
This clue is a double definition.
'service' is the first definition.
(I've seen this before)
'tax' is the second definition.
(duty can mean a tax or toll)
(Other definitions for duty that I've seen before include "Government tax" , "Is one bound" , "Cancel (a law)" , "form of tax" , "Obligation; levy" .)