Gadget needed for an American soldier's mode of operation (5)
I believe the answer is:
gismo
'gadget' is the definition.
(I know that gadget can be written as gismo)
'an american soldier's mode of operation' is the wordplay.
'an american soldier's' becomes 'GIs' (informal term for a US soldier).
'mode of operation' becomes 'mo' (abbreviation for modus operandi).
'gis'+'mo'='GISMO'
'needed for' is the link.
(Other definitions for gismo that I've seen before include "Useful gadget" , "mechanical device?" , "Fancy device" , "Doofer" , "Device of American" .)