Part of the mouth (3)
'part of the mouth' is the definition.
(lips are part of the mouth)
This is all the clue.
(Other definitions for lip that I've seen before include "giving such insincere service" , "In slang, impudent talk" , "Rim of an orifice" , "Brim" , "It can be read" .)
'part of the mouth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roof that I've seen before include "Top covering of structure" , "Top outside covering" , "Housetop" , "Upper permitted limit" , "Part of mouth" .)