Blabbermouth, snitch (8)
I believe the answer is:
telltale
This clue is a double definition.
'blabbermouth' is the first definition.
(I know that blabbermouth can be written as telltale)
'snitch' is the second definition.
(I've seen this before)
(Other definitions for telltale that I've seen before include "Gossip-monger" , "Revealer of secrets" , "One revealing secrets" , "Revelatory - blabbermouth" , "Blabbermouth, snitch" .)