Horrible green Yankee spirit (6)
I believe the answer is:
energy
'spirit' is the definition.
(I know that energy is a type of spirit)
'horrible green yankee' is the wordplay.
'horrible' indicates an anagram.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Vivacity" , "Capacity for action" , "Vigour, animation" , "Pep" , "Vitality, get-up-and-go" .)