Very sad US soldier in cart, reversing (6)
I believe the answer is:
tragic
'very sad' is the definition.
(tragic can mean very sad or unfortunate)
'us soldier in cart reversing' is the wordplay.
'us soldier' becomes 'GI' (GI can mean an American soldier).
'in' indicates putting letters inside.
'reversing' says the letters should be written backwards.
'cart' written backwards gives 'trac'.
'gi' placed within 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Mournful" , "Pathetic" , "Dire" , "Shocking" , "Disastrous, desperately sad" .)