Mostly wise over fossil fuel (3)
I believe the answer is:
gas
'fossil fuel' is the definition.
(I've seen this before)
'mostly wise over' is the wordplay.
'mostly' means to remove the last letter.
'wise' becomes 'sage' ('sage' can be similar in meaning to 'wise').
'over' says the letters should be written in reverse.
'sage' with its last letter taken away is 'sag'.
'sag' in reverse letter order is 'GAS'.
(Other definitions for gas that I've seen before include "See 2" , "American fuel" , "Eg, nitrogen" , "Hydrogen or oxygen, say" , "person who's amusing ..." .)