Break promise about green energy (6)
I believe the answer is:
renege
'break promise' is the definition.
(as in reneging on a deal)
'about green energy' is the wordplay.
'about' is an anagram indicator.
'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 "Fail to carry out a commitment" , "Fail to fulfil a promise or obligation" , "Go back on one's word, break promise" , "Fail to follow suit at cards" , "Fail to follow suit or fulfil a promise" .)