Writing on the wall round gents (4)
I believe the answer is:
omen
'writing on the wall' is the definition.
(both can mean a sign of future misfortune)
'round gents' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'gents' becomes 'men' (gent can mean a man).
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "Warning token" , "writing on the wall" , "Straw in the wind" , "Portent of things to come" , "Sign of a future event, good or bad" .)