Turn green with unknown vigour (6)
I believe the answer is:
energy
'vigour' is the definition.
('energy' can be a synonym of 'vigour')
'turn green with unknown' is the wordplay.
'turn' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Power; capacity for work" , "Zest" , "Go, zip" , "Fuel" , "Healthy capacity for vigorous activity" .)