Change one's mind about green energy (6)
I believe the answer is:
renege
'change one's mind' is the definition.
(I've seen this before)
'about green energy' is the wordplay.
'about' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'green' is an anagram of 'reneg'.
'reneg'+'e'='RENEGE'
(Other definitions for renege that I've seen before include "South American cowboy" , "Go back on an obligation" , "Go back (on promise etc)" , "Fail to follow suit or fulfil a promise" , "In card games, revoke" .)