Spent energy on fine garden party (6)
I believe the answer is:
effete
'spent' is the definition.
(thesaurus)
'energy on fine garden party' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'on' says to put letters next to each other.
'fine' becomes 'f' (abbreviation).
'garden party' becomes 'fete' (I've seen this before).
'e'+'f'+'fete'='EFFETE'
(Other definitions for effete that I've seen before include "Enfeebled" , "Worn-out, decadent" , "Lacking in power" , "Over-refined; weak" , "Feeble, having lost vitatlity" .)