Prevent a former soldier keeping rifle, at first (5)
I believe the answer is:
avert
'prevent' is the definition.
(averting is a kind of preventing)
'a former soldier keeping rifle at first' is the wordplay.
'former soldier' becomes 'vet' (short for veteran).
'keeping' indicates putting letters inside.
'at first' suggests taking the first letters.
The first letter of 'rifle' is 'r'.
'vet' going around 'r' is 'vert'.
'a'+'vert'='AVERT'
(Other definitions for avert that I've seen before include "Turn away from, say gaze" , "Turn away, prevent" , "Turn away (ones eyes)" , "Prevent, ward off" , "Turn aside or ward off" .)