Fuel unaffected by wind (7,3)
I believe the answer is:
natural gas
'fuel' is the definition.
(I know that natural gas is a type of fossil fuel)
'unaffected by wind' is the wordplay.
'unaffected' becomes 'natural' (similar in meaning).
'by' means one lot of letters go next to another.
'wind' becomes 'gas' (synonyms).
'natural'+'gas'='NATURAL GAS'
(Other definitions for natural gas that I've seen before include "Fuel from eg 2 down" , "Energy source from underground" , "Fuel from underground" .)