It should be sort of green, years on? (6)
I believe the answer is:
energy
'it should' is the definition.
The definition suggests a singular noun which matches the answer.
'sort of green years on?' is the wordplay.
'sort of' indicates anagramming the letters.
'years' becomes 'y' (abbreviation).
'on?' says to put letters next to each other.
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
'be' is the link.
Can you help me to learn more?
(Other definitions for energy that I've seen before include "Zest" , "Power or vigorous activity" , "Gusto" , "Verve" , "Exertion of force" .)