Breach of secrecy (4)
I believe the answer is:
'breach of secrecy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leak that I've seen before include "Unauthorised disclosure of confidential information" , "Accidental hole allowing liquid to escape" , "Unwanted discharge" , "(Politically motivated) seepage?" , "Spill secrets" .)