Sensational turn, OK? (8)
I believe the answer is:
knockout
This clue is a double definition.
'sensational' is the first definition.
'turn ok?' is the second definition.
(ko is short for knockout)
(Other definitions for knockout that I've seen before include "Winning blow" , "Render senseless" , "Progressively narrowing competition" , "Blow that deprives boxer of his senses" , "stunner" .)