It's very sad. The vehicle overturned trapping the soldier (6)
I believe the answer is:
tragic
'it's very sad' is the definition.
(tragic can mean very sad or unfortunate)
'vehicle overturned trapping the soldier' is the wordplay.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'overturned' says the letters should be written in reverse.
'trapping' is an insertion indicator.
'the soldier' becomes 'GI' (GI can mean a US soldier).
'cart' backwards is 'trac'.
'trac' going around 'gi' is 'TRAGIC'.
'the' is the link.
(Other definitions for tragic that I've seen before include "Deeply distressing" , "Awful" , "Terribly sad and distressing" , "Extremely sad and distressing" , "Dire" .)