Simple Australian tree chopped up (7)
I believe the answer is:
austere
'simple' is the definition.
(I've seen this in another clue)
'australian tree chopped up' is the wordplay.
'australian' becomes 'aus' (common abbreviation).
'chopped up' is an anagram indicator.
'tree' with letters rearranged gives 'tere'.
'aus'+'tere'='AUSTERE'
(Other definitions for austere that I've seen before include "Without luxury or ornament" , "severely self-disciplined" , "Severe in appearance" , "Severe, lacking comforts" , "not of luxury type" .)