Fuel soldier fellow with bit of fish (8)
I believe the answer is:
paraffin
'fuel' is the definition.
(paraffin is a kind of fuel)
'soldier fellow with bit of fish' is the wordplay.
'soldier' becomes 'para' (paratrooper).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'with' is a charade indicator (letters next to each other).
'bit of fish' becomes 'fin' (fish have fins).
'para'+'f'+'fin'='PARAFFIN'
(Other definitions for paraffin that I've seen before include "Possibly oil" , "Kerosene" , "Waxy flammable substance" , "Fuel oil obtained from petroleum or coal" , "Flammable wax" .)