Divulge a secret (4)
'divulge a secret' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for blab that I've seen before include "Spill the beans" , "24 ac secrets in talk" , "Let slip" , "Divulge secret information" , "Give away secrets" .)
(Other definitions for tell that I've seen before include "Narrate; inform" , "blow the whistle" , "William .... was legendary Swiss hero-archer" , "Count - recount" , "Relate; Swiss hero" .)
(Other definitions for blow the gaff that I've seen before include "Reveal a secret" , "Blurt out secret" .)
(Other definitions for let on that I've seen before include "Allow to board, or pretend" , "Divulge information" , "Spill the beans" , "Reveal information" , "Be revealing" .)