Sweet, one that’s rich with key energy (6)
I believe the answer is:
toffee
'sweet' is the definition.
(toffee is a kind of sweet)
'one that's rich with key energy' is the wordplay.
'one that's rich' becomes 'toff' (toff can mean a rich or upper-class person).
'with' is a charade indicator (letters next to each other).
'key' becomes 'e' (musical key).
'energy' becomes 'e' (physics symbol as in E=mc2).
'toff'+'e'+'e'='TOFFEE'
(Other definitions for toffee that I've seen before include "Sugar/butter sweet" , "sticky stuff" , "Chewy stuff" , "Sweet made with heated butter and sugar" , "Chewy sweet" .)