Behave ostentatiously (4,3)
I believe the answer is:
show off
'behave ostentatiously' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for show off that I've seen before include "Flaunt - exhibitionist" , "In conceitedness" , "Behave ostentatiously" , "Display ostentatiously, try to impress" , "Ostentatious person" .)