Unauthorised disclosure (4)
I believe the answer is:
leak
'unauthorised disclosure' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for leak that I've seen before include "Accidental hole that allows fluid to escape" , "Seep; disclose" , "Divulge" , "(Politically motivated) seepage?" , "Disclose (secrets)" .)