Soldier's doctor's thingummy (5)
I believe the answer is:
gismo
'thingummy' is the definition.
(I know that thingummy can be written as gismo)
'soldier's doctor's' is the wordplay.
'soldier's' becomes 'GI's' (GI can mean a US soldier).
'doctor' becomes 'mo' (medical officer).
'gis'+'mo'='GISMO'
(Other definitions for gismo that I've seen before include "Ingenious gadget" , "mechanical device?" , "Useful gadget" , "Fancy device" , "Thingumajig - gadget" .)