Crossword Genius

Sweet, one that’s rich with key energy (6)

Ross

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" .)

I've seen this clue in The Times.
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