Old soldiers get sign (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'
'get' is the link.
(Other definitions for omen that I've seen before include "Foreshadow" , "Forewarning, harbinger" , "Sign of likely future event" , "Portent of things to come" , "Harbinger, prophetic sign" .)