He leaves the US soldier in broken-down car -- that's sad (6)
I believe the answer is:
tragic
'sad' is the definition.
('tragic' can be similar in meaning to 'sad')
'he leaves the us soldier in broken-down car' is the wordplay.
'leaves' is a deletion indicator.
'us soldier' becomes 'gi' (I've seen this in other clues).
'in' indicates putting letters inside.
'broken down' indicates anagramming the letters.
'the' with 'he' removed is 't'.
'car' anagrammed gives 'rac'.
'gi' placed into 'rac' is 'ragic'.
't'+'ragic'='TRAGIC'
'that's' is the link.
(Other definitions for tragic that I've seen before include "Dire" , "Resulting in great loss" , "Causing extreme distress or sorrow" , "Ill-fated" , "Greatly distressing" .)