Mouth, slangily (3)
I believe the answer is:
yap
'mouth slangily' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yap that I've seen before include "Pay up for shrill bark (3" , "Chatter at length" , "Bark incessantly" , "Complain shrilly" , "Natter; small dog's bark" .)