Writing on the wall up in Stone Mountain (4)
I believe the answer is:
omen
'writing on the wall' is the definition.
(I've seen this before)
'up in stone mountain' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates a hidden word.
'nemo' can be found hidden inside 'stone mountain'.
'nemo' written backwards gives 'OMEN'.
(Other definitions for omen that I've seen before include "Augury, portent" , "premonition" , "Portent of things to come" , "Warning token" , "Gloomy sign" .)