Very sad soldier in vehicle going back uphill (6)
I believe the answer is:
tragic
'very sad' is the definition.
(I've seen this before)
'soldier in vehicle going back uphill' is the wordplay.
'soldier' becomes 'gi' (GI can mean a US soldier).
'in' means one lot of letters goes inside another.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'going back uphill' shows that the letters should be reversed in order (in a down clue, letters go up).
'cart' in reverse letter order is 'trac'.
'gi' put inside 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Characterised by extreme distress or sorrow" , "Very sad; Schubert's 4th symphony nickname" , "Ill-fated" , "Unfortunate" , "Extremely sad; Mahler's 6th nickname" .)