Ostentatious display cut — power intervenes (6)
I believe the answer is:
splash
'ostentatious display' is the definition.
(as in making a splash)
'cut power intervenes' is the wordplay.
'cut' becomes 'slash' (slashing is a kind of cutting).
'power' becomes 'p' (symbol in physics).
'intervenes' is an insertion indicator.
'slash' placed around 'p' is 'SPLASH'.
(Other definitions for splash that I've seen before include "Dash with liquid" , "Drop" , "Small amount of something liquid" , "Watery sound" , "Effect of jumping into water" .)