Part of mouth that's hit by very angry person (4)
I believe the answer is:
roof
This might be a double definition.
'part of mouth' is the first definition.
(I've seen this before)
'hit by very angry person' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for roof that I've seen before include "Cover for building" , "Upper permitted limit" , "Covering of house" , "Topping structure" , "Top of car or house" .)