Zeal of green revolutionary ending in calamity (6)
I believe the answer is:
energy
'zeal' is the definition.
(I've seen this before)
'green revolutionary ending in calamity' is the wordplay.
'revolutionary' indicates anagramming the letters.
'ending in' suggests the final letters.
The final letter of 'calamity' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
'of' acts as a link.
(Other definitions for energy that I've seen before include "Vigour, vim, vitality" , "Fuel" , "Seventeen forty six defeat for Bonnie Prince Charlie" , "Power - drive" , "Capacity for action" .)