Go when green has changed? Start off yes (6)
I believe the answer is:
energy
'go' is the definition.
(I've seen this before)
'green has changed? start off yes' is the wordplay.
'has changed?' indicates anagramming the letters.
'start off' indicates taking the first letters.
The first letter of 'yes' is 'y'.
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
'when' acts as a link.
(Other definitions for energy that I've seen before include "Vigour, spirit" , "Pep" , "Power or vigorous activity" , "Vigour, power" , "Dynamism" .)