Power is terribly green and variable (6)
I believe the answer is:
energy
'power' is the definition.
(synonyms)
'terribly green and variable' is the wordplay.
'terribly' is an anagram indicator.
'and' is a charade indicator (letters next to each other).
'variable' becomes 'y' (algebra).
'green' is an anagram of 'energ'.
'energ'+'y'='ENERGY'
'is' acts as a link.
(Other definitions for energy that I've seen before include "Seventeen forty six defeat for Bonnie Prince Charlie" , "Pep" , "Power; capacity for work" , "'Vigour, vitality (6)'" , "Go green, possibly with envy, ultimately" .)