In favour of old tax system (7)
I believe the answer is:
process
'system' is the definition.
(thesaurus)
'in favour of old tax' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'old tax' becomes 'cess' (I've seen this before. I am not sure about the 'old' bit.).
'pro'+'cess'='PROCESS'
Can you help me to learn more?
(Other definitions for process that I've seen before include "The case" , "Series of steps achieving a particular end" , "Procedure" , "Corpses in series of steps" , "Series of actions with a single purpose" .)