Venomous sort of chatterbox losing face (7)
I believe the answer is:
rattler
'venomous sort' is the definition.
(I know that rattlesnakes are venomous)
'chatterbox losing face' is the wordplay.
'chatterbox' becomes 'prattler' (prattle can mean to talk a lot).
'losing face' means to remove the first letter.
'prattler' with its initial letter taken away is 'RATTLER'.
'of' acts as a link.
(Other definitions for rattler that I've seen before include "Dangerous snake" , "US snake (colloq.)" , "old crate" , "(it crawls?)" , "American snake (informal)" .)