Nick isn't finished with fossil fuel (3)
I believe the answer is:
gas
'fossil fuel' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "Substance like air" , "eg Helium" , "Chatter (colloq.)" , "Gossip" , "Nitrogen, say" .)