Stupid Greek character hiding in tree (5)
I believe the answer is:
apish
'stupid' is the definition.
(thesaurus)
'greek character hiding in tree' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'hiding in' is an insertion indicator.
'tree' becomes 'ash' (ash tree is a kind of tree).
'pi' put inside 'ash' is 'APISH'.
(Other definitions for apish that I've seen before include "Foolish; simian" , "Foppish" , "Stupid" , "Silly" , "Foolish; like a monkey" .)