Sudden insight (8)
I believe the answer is:
epiphany
'sudden insight' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for epiphany that I've seen before include "Church festival" , "Moment of sudden revelation (involving three wise men from the east?)" , "Sudden realisation" , "Christian feast of showing Christ to Magi" , "Sudden insight" .)