Narrate story given by blabbermouth (8)
I believe the answer is:
telltale
'blabbermouth' is the definition.
('telltale' can be a synonym of 'blabbermouth')
'narrate story given' is the wordplay.
'narrate' becomes 'tell' ('tell' can be a synonym of 'narrate').
'story' becomes 'tale' ('tale' can be a synonym of 'story').
'given' is a charade indicator (letters next to each other).
'tell'+'tale'='TELLTALE'
'by' is the link.
(Other definitions for telltale that I've seen before include "Blabber-mouth, can't keep a secret" , "Revealing or indicating something, like blabbermouth" , "One revealing secrets" , "Betraying" , "Outward indication of something concealed" .)