Rooftop wall (7)
I believe the answer is:
parapet
'rooftop wall' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for parapet that I've seen before include "form of defence" , "wall, protective one" , "Fortification consisting of a low wall" , "Low wall round edge of balcony" , "Defensive line" .)