Ascetic American hiding in a tree, dishevelled (7)
I believe the answer is:
austere
'ascetic' is the definition.
(I know that ascetic can be written as austere)
'american hiding in a tree dishevelled' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'hiding in' indicates putting letters inside.
'dishevelled' indicates an anagram.
'a'+'tree'='atree'
'atree' with letters rearranged gives 'atere'.
'us' inserted into 'atere' is 'AUSTERE'.
(Other definitions for austere that I've seen before include "Plain, cold" , "severely self-disciplined" , "'Spartan, simple (7)'" , "Severe, lacking comforts" , "Strict in appearance" .)