Sad soldier in an overturned vehicle (6)
I believe the answer is:
tragic
'sad' is the definition.
(similar in meaning)
'soldier in an overturned vehicle' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'in' is an insertion indicator.
'an overturned' is a reversal indicator.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'cart' reversed gives 'trac'.
'gi' put within 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Ill-fated" , "like King Lear" , "Causing extreme distress or sorrow" , "Dire" , "Pitiable" .)