Smart-mouthed (5)
I believe the answer is:
sassy
'smart-mouthed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sassy that I've seen before include "Pert" , "Impertinent (US colloq.)" , "Cheeky American" , "Bold, spirited, cheeky (Amer.)" , "(In US) saucy" .)