Fuel soldier dropped: horrible smell rising (8)
I believe the answer is:
paraffin
'fuel' is the definition.
(paraffin is a kind of fuel)
'soldier dropped horrible smell rising' is the wordplay.
'soldier' becomes 'para' (paratrooper).
'dropped horrible smell' becomes 'niff' (I am not sure about this - if you are sure you should believe this answer much more).
'rising' is a reversal indicator (in a down clue, letters go up).
'niff' backwards is 'ffin'.
'para'+'ffin'='PARAFFIN'
Can you help me to learn more?
(Other definitions for paraffin that I've seen before include "Kerosene" , "Possibly oil" , "Petroleum product" , "Fuel oil obtained from petroleum or coal" , "Kerosine" .)