Go green, being indisposed to say why (6)
I believe the answer is:
energy
'go' is the definition.
(thesaurus)
'green being indisposed to say why' is the wordplay.
'being indisposed' is an anagram indicator.
'to say why' becomes 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Power or vigorous activity" , "Fuel" , "Power - drive" , "Vigour, power" , "Exertion of force" .)