Sign to old soldiers (4)
I believe the answer is:
omen
'sign' 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'
'to' is the link.
(Other definitions for omen that I've seen before include "Prophetic significance" , "Sign of things to come" , "Warning - sign" , "premonition" , "Portent, prophetic indicator" .)