Act of defiance (4)
I believe the answer is:
dare
'act of defiance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dare that I've seen before include "Be brave enough" , "Risk; challenge" , "Be courageous enough to try or do" , "Challenge to do something foolhardy" , "Take a chance - read about it" .)