Protruding part of the jaw (4)
I believe the answer is:
chin
'protruding part of the jaw' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chin that I've seen before include "Facial feature" , "narrow opening" , "Punch to the face" , "Protruding part of lower jaw" , "What's repeatedly required for a toast" .)