Where American casualties may be tense still (5)
I believe the answer is:
inert
'still' is the definition.
(not moving)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for inert that I've seen before include "Incapable of acting, like some gas" , "Nitre (anag.)" , "(Chem.) not reactive" , "dead" , "Torpid" .)