Strikingly bright or ostentatious (5)
I believe the answer is:
showy
I believe this clue is a double definition.
'strikingly bright' is the first definition.
(I've seen this before)
'ostentatious' is the second definition.
(similar in meaning)
(Other definitions for showy that I've seen before include "Strikingly colourful" , "Colourful, ostentatious" , "Ostentatious, bright" , "Strikingly bright" , "Flamboyant" .)