Sad soldier in cart crash (6)
I believe the answer is:
tragic
'sad' is the definition.
(similar in meaning)
'soldier in cart crash' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'in' means one lot of letters goes inside another.
'crash' is an anagram indicator.
'cart' anagrammed gives 'trac'.
'gi' placed inside 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Dreadful" , "Pitiable" , "Pathetic" , "Disastrous, appalling" , "Dire" .)