Reversal of decline in fossil fuel (3)
I believe the answer is:
gas
'fossil fuel' is the definition.
(I've seen this before)
'reversal of decline' is the wordplay.
'reversal of' is a reversal indicator.
'decline' becomes 'sag' (I've seen this before).
'sag' written backwards gives 'GAS'.
'in' acts as a link.
(Other definitions for gas that I've seen before include "Chatter (colloq.)" , "Americans' petrol" , "idle chit-chat" , "Petrol (USA)" , "Carbon dioxide, say" .)