Genuinely devoid of energy in comeback (5)
I believe the answer is:
rally
'comeback' is the definition.
(thesaurus)
'genuinely devoid of energy' is the wordplay.
'genuinely' becomes 'really' (I've seen this before).
'devoid of' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'really' with 'e' taken out is 'RALLY'.
'in' is the link.
(Other definitions for rally that I've seen before include "Get-together" , "Recover; car race" , "Mass political meeting" , "Recuperate in gathering" , "Regain one's strength" .)