Old soldiers warning (4)
I believe the answer is:
omen
'warning' is the definition.
(I've seen this before)
'old soldiers' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldiers' becomes 'men' (I've seen this before**).
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "Harbinger, prophetic sign" , "premonition" , "Prognostication" , "Gloomy sign" , "Straw in the wind" .)