Like William Wallace's outdated tax, this is complicated (8)
I believe the answer is:
scottish
'like william wallace's' is the definition.
'outdated tax this is complicated' is the wordplay.
'outdated tax' becomes 'scot' (archaic term for a tax).
'is complicated' indicates an anagram.
'this' with letters rearranged gives 'tish'.
'scot'+'tish'='SCOTTISH'
(Other definitions for scottish that I've seen before include "From north of the border" , "Caledonian" , "So stitch up to be like Angus or Jock" , "Costs hit this nationality badly" , "northern folk" .)