Judgement last month on energy (6)
I believe the answer is:
decree
'judgement' is the definition.
(thesaurus)
'last month on energy' is the wordplay.
'last month' becomes 'dec' (short for December, last month of the year).
'on' becomes 're' ('on' can be mean 'with reference to').
'energy' becomes 'e' (physics symbol as in E=mc2).
'dec'+'re'+'e'='DECREE'
(Other definitions for decree that I've seen before include "Official order of ruling" , "Direct" , "Official edict" , "Official order that has the force of law" , "Recede (anag.)" .)