Eat well, but don’t eat with energy (5)
I believe the answer is:
feast
'eat well' is the definition.
(I've seen this in another clue)
'don't eat with energy' is the wordplay.
'don't eat' becomes 'fast' (fasting is not eating).
'with' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'fast' enclosing 'e' is 'FEAST'.
'but' acts as a link.
(Other definitions for feast that I've seen before include "Sumptuous meal" , "Abundant meal" , "Huge nosh-up" , "Junket" , "Celebration" .)