Sign from old people (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'old people' is the wordplay.
'old' becomes 'o' (abbreviation).
'people' becomes 'men' (men are people).
'o'+'men'='OMEN'
'from' acts as a link.
(Other definitions for omen that I've seen before include "Gloomy sign" , "Harbinger, portent" , "Portent of evil" , "Augur" , "Sign of a future event, good or bad" .)