Tax is sound old measure (6)
I believe the answer is:
excise
'tax' is the definition.
(I know that excise is a more specific form of the action tax)
'is sound old measure' is the wordplay.
I cannot really understand how this works, but
'is' is within the answer.
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
'measure' could be 'e' (e is a kind of measure) and 'e' is present in the answer.
A single letter 'c' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for excise that I've seen before include "A tax to be cut out, perhaps" , "Tax on certain goods and commodities" , "Customs" , "that wretches hatefully collect" , "Expunge the tax?" .)