Turning green with unknown vitality (6)
I believe the answer is:
energy
'vitality' is the definition.
(I've seen this before)
'turning green with unknown' is the wordplay.
'turning' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'green' is an anagram of 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Dynamism" , "life" , "Healthy capacity for vigorous activity" , "Vigour, power" , "Vitality, get-up-and-go" .)