Sign from old soldiers (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'old soldiers' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldiers' becomes 'men' (men can mean a group of soldiers).
'o'+'men'='OMEN'
'from' acts as a link.
(Other definitions for omen that I've seen before include "Sign of likely future event" , "Straw in the wind" , "Gloomy sign" , "Foreboding" , "Warning - sign" .)