Bragging (8)
'bragging' is the definition.
('boast' can be a synonym of 'brag')
This is all the clue.
(Other definitions for boasting that I've seen before include "Talking conceitedly" , "Bragging, showing off" , "Preening" .)
(Other definitions for boastful that I've seen before include "Given to blowing one's own trumpet" , "Loudly conceited" , "Vainglorious" , "Talking with excessive pride in oneself" , "Immodest" .)
'bragging' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for mouth that I've seen before include "Oral cavity" , "Opening; say" , "Where a river enters the sea" , "chops" , "Gob" .)