Green reforms with unknown power (6)
I believe the answer is:
energy
'power' is the definition.
(synonyms)
'green reforms with unknown' is the wordplay.
'reforms with' indicates anagramming the letters.
'unknown' becomes 'y' (algebra).
'green' mixed up with 'y' gives 'ENERGY'.
(Other definitions for energy that I've seen before include "Zeal" , "Power - drive" , "Exertion of force" , "Capacity for action" , "Vitality, get-up-and-go" .)