American soldier's method of operating a device (5)
I believe the answer is:
gismo
'a device' is the definition.
(I've seen this before)
'american soldier's method' is the wordplay.
'american soldier's' becomes 'GI's' (informal term for a US soldier).
'method' becomes 'MO' (modus operandi, a particular method).
'gis'+'mo'='GISMO'
'of operating' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gismo that I've seen before include "Doofer" , "mechanical device?" , "Thingumajig - gadget" , "Gadget (colloq.)" , "Fancy device" .)