Go strangely green with envy ultimately (6)
I believe the answer is:
energy
'go' is the definition.
(I've seen this before)
'strangely green with envy ultimately' is the wordplay.
'strangely' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'ultimately' says to take the final letters.
The final letter of 'envy' is 'y'.
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Healthy capacity for vigorous activity" , "Power or vigorous activity" , "Vigour, spirit" , "Capacity for action" , "Power - drive" .)