Old people providing warning (4)
I believe the answer is:
omen
'warning' is the definition.
(I've seen this before)
'old people' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'people' becomes 'men' (man is a kind of person).
'o'+'men'='OMEN'
'providing' is the link.
(Other definitions for omen that I've seen before include "Portent of evil" , "Foreshadow" , "Auspice" , "Prophetic significance" , "Sign of something to come" .)