To soldiers, one source of danger (6)
I believe the answer is:
menace
'source of danger' is the definition.
(I've seen this before)
'to soldiers one' is the wordplay.
'to soldiers' becomes 'men' (men can mean a group of soldiers).
'one' becomes 'ace' (ace has a value of one in some card games).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threat, show of hostility" , "Intimidate" , "Intimidation" , "Threaten with violence" , "serious nuisance" .)