Blab a secret (4)
I believe the answer is:
tell
'blab a secret' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tell that I've seen before include "Relate (a story)" , "William . . . . was a legendary Swiss archer" , "Notify" , "Count - recount" , "Peach" .)