Green fuel must go up, suppressing energy (4)
I believe the answer is:
sage
'green' is the definition.
(sage green is a kind of green)
'fuel must go up suppressing energy' is the wordplay.
'fuel' becomes 'gas' (gas is a kind of fuel).
'must go up' says the letters should be written backwards.
'suppressing' says to put letters next to each other (in a down clue, some letters go on top of others).
'energy' becomes 'e' (physics symbol as in E=mc2).
'gas' reversed gives 'sag'.
'sag'+'e'='SAGE'
(Other definitions for sage that I've seen before include "Clever" , "Solon, for instance" , "Salvia" , "Velvety-leaved herb" , "cheesemongers associate with Derby" .)