Energy to leave self-regard (3)
I believe the answer is:
ego
'self-regard' is the definition.
(I've seen this in another clue)
'energy to leave' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'to leave' becomes 'go' (to go is to depart or leave).
'e'+'go'='EGO'
(Other definitions for ego that I've seen before include "Conscious self" , "Self-esteem or self-importance" , "Sense of identity" , "One's image of oneself" , "Morale" .)