Self-esteem and energy to depart (3)
I believe the answer is:
ego
'self-esteem' is the definition.
(ego can mean one's sense of self)
'energy to depart' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'to depart' becomes 'go' (synonyms).
'e'+'go'='EGO'
'and' acts as a link.
(Other definitions for ego that I've seen before include "(Self-assertive) personality" , "One's self-image" , "Conscious mind" , "See 21 Down" , "In Virgil I" .)