Ordinary guys writing on the wall (4)
I believe the answer is:
omen
'writing on the wall' is the definition.
(I've seen this before)
'ordinary guys' is the wordplay.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'guys' becomes 'men'.
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "Gloomy sign" , "Sign of a future event, good or bad" , "Harbinger, augury" , "Sign of something about to happen" , "Sign of likely future event" .)