let the cat out of the bag (8)
'let the cat out of the bag' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for blab that I've seen before include "Tell tales" , "Tells tales" , "Let the cat out of the bag" , "Let slip" , "Spill the beans" .)
(Other definitions for disclose that I've seen before include "Betray" , "Make (secret) known" , "Sold ices (anag)" , "Divulge, reveal" , "broadcast" .)