Force green reforms on party, finally (6)
I believe the answer is:
energy
'force' is the definition.
(energy is a kind of force)
'green reforms on party finally' is the wordplay.
'reforms' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'finally' suggests the final letters.
The final letter of 'party' is 'y'.
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Go, zip" , "Verve" , "Power; capacity for work" , "Zeal" , "Vigour, spirit" .)