Organ of body, lacking energy, managed? Excellent (8)
I believe the answer is:
splendid
'excellent' is the definition.
(I know that excellent can be written as splendid)
'organ of body lacking energy managed?' is the wordplay.
'organ of body' becomes 'spleen' (the spleen is an organ).
'lacking' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'managed?' becomes 'did' ('do' can mean 'manage' as in 'do without').
'spleen' with 'e' taken out is 'splen'.
'splen'+'did'='SPLENDID'
(Other definitions for splendid that I've seen before include "Magnificent, very impressive" , "Noble" , "Magnificent, brilliant" , "Grand" , "Wicked" .)