Distressing cart accident involves soldier (6)
I believe the answer is:
tragic
'distressing' is the definition.
(I've seen this before)
'cart accident involves soldier' is the wordplay.
'cart accident' becomes 'trac' (I can't justify this - if you can you should give a lot more credence to this answer).
'involves' is an insertion indicator.
'soldier' becomes 'GI' (GI can mean a US soldier).
'trac' enclosing 'gi' is 'TRAGIC'.
Can you help me to learn more?
(Other definitions for tragic that I've seen before include "Calamitous, causing great sadness" , "Intensely sad and distressing" , "Causing extreme distress or sorrow" , "Disastrous, appalling" , "Greatly distressing" .)