Gaudy insect on outside of tree (6)
I believe the answer is:
flashy
'gaudy' is the definition.
('flashy' can be a synonym of 'gaudy')
'insect on outside of tree' is the wordplay.
'insect' becomes 'fly' (fly is a kind of insect).
'on outside of' means one lot of letters goes inside another.
'tree' becomes 'ash' (ash is a kind of tree).
'fly' enclosing 'ash' is 'FLASHY'.
(Other definitions for flashy that I've seen before include "Showy but empty; Has fly (anag.)" , "Flamboyantly attractive" , "Loud" , "Cheap and ostentatious" , "Garish" .)