US soldier’s short time gadget (5)
I believe the answer is:
gismo
'gadget' is the definition.
(I know that gadget can be written as gismo)
'us soldier's short time' is the wordplay.
'us soldier's' becomes 'GI's' (GI can mean an American soldier).
'short time' becomes 'mo' (as in 'just a mo').
'gis'+'mo'='GISMO'
(Other definitions for gismo that I've seen before include "Thingumajig - gadget" , "Ingenious gadget" , "Fancy device" , "Useful gadget" , "Device of American" .)