Turn green with envy in the end — that's life! (6)
I believe the answer is:
energy
'that's life' is the definition.
(energy is a kind of life)
'turn green with envy in the end' is the wordplay.
'turn' indicates anagramming the letters.
'with' says to put letters next to each other.
'in the end' indicates one should take the final letters.
The last letter of 'envy' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Vitality, power" , "Pep" , "Go, zip" , "Gusto" , "Power - drive" .)