Plant to get fossil fuel reportedly? Not entirely raving mad (8)
I believe the answer is:
kohlrabi
'plant' is the definition.
(vegetable plant)
'fossil fuel reportedly? not entirely raving mad' is the wordplay.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'reportedly?' indicates a 'sounds like' (homophone) clue (I've seen 'reportedly' mean this).
'not entirely' means to remove the last letter.
'raving mad' becomes 'rabid' (rabid can mean violently angry).
'coal' is a homophone of 'kohl'.
'rabid' with its final letter taken away is 'rabi'.
'kohl'+'rabi'='KOHLRABI'
'to get' acts as a link.
(Other definitions for kohlrabi that I've seen before include "Variety of cabbage like a turnip" , "leaves to be consumed" , "vegetable" .)