American sitting in a tree, unusually ascetic (7)
I believe the answer is:
austere
'ascetic' is the definition.
('austere' can be a synonym of 'ascetic')
'american sitting in a tree unusually' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'sitting in' means one lot of letters goes inside another.
'unusually' is an anagram indicator.
'a'+'tree'='atree'
'atree' is an anagram of 'atere'.
'us' inserted within 'atere' is 'AUSTERE'.
(Other definitions for austere that I've seen before include "Severe in appearance" , "Serious" , "not of luxury type" , "Without luxury or ornament" , "Severe in manner" .)