Fuel unaffected by gossip (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 gossip' is the wordplay.
'unaffected' becomes 'natural' (synonyms).
'by' says to put letters next to each other.
'gossip' becomes 'gas' (I've seen this before).
'natural'+'gas'='NATURAL GAS'
(Other definitions for natural gas that I've seen before include "Fuel from eg 2 down" , "Fuel from underground" , "Energy source from underground" .)