Green energy, reason to cut fossil fuel (10)
I believe the answer is:
ecological
'green' is the definition.
(I've seen this before)
'energy reason to cut fossil fuel' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'reason' becomes 'logic' (I've seen this before**).
'to cut' means one lot of letters goes inside another.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'logic' put within 'coal' is 'cological'.
'e'+'cological'='ECOLOGICAL'
(Other definitions for ecological that I've seen before include "Gaelic cool (anag.)" , "Green" .)