Not subject to taxation (6)
I believe the answer is:
exempt
'not subject to taxation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for exempt that I've seen before include "Freed from obligation, tax etc" , "Immune - absolve" , "free from tax, say" , "Excused - let off" , "Free (from taxes, etc)" .)