Ill-fated US soldier trapped in overturned wagon (6)
I believe the answer is:
tragic
'ill-fated' is the definition.
(I've seen this in another clue)
'us soldier trapped in overturned wagon' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'trapped in' is an insertion indicator.
'overturned' says the letters should be written in reverse.
'wagon' becomes 'cart' (cart is a kind of wagon).
'cart' backwards is 'trac'.
'gi' going within 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Very sad; Schubert's 4th symphony nickname" , "Unfortunate" , "Mournful" , "Extremely sad and distressing" , "Calamitous, causing great sadness" .)