Ostentatious moment (5)
I believe the answer is:
flash
I believe this clue is a double definition.
'ostentatious' is the first definition.
(I've seen this before)
'moment' is the second definition.
(I know that flash is a type of moment)
(Other definitions for flash that I've seen before include "short space of time" , "Scintillate" , "emblem on uniform" , "Ostentatious" , "Camera fitment" .)