Oomph
'oomph' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for energy that I've seen before include "Vitality, get-up-and-go" , "Power; capacity for work" , "life" , "Vigour, vim, vitality" , "Go green, possibly with envy, ultimately" .)
(Other definitions for zest that I've seen before include "Zing" , "Orange or lemon peel" , "Enthusiasm or outer skin of citrus fruit" , "Piquancy (eg from lemon peel)" , "Outer layer of citrus peel" .)