Old man is full of energy? Exactly! (4)
I believe the answer is:
dead
'exactly' is the definition.
(eg in phrases like 'you're dead right')
'old man is full of energy?' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'is full of' is an insertion indicator.
'energy?' becomes 'e' (physics symbol as in E=mc2).
'dad' enclosing 'e' is 'DEAD'.
(Other definitions for dead that I've seen before include "Deceased; numb" , "Not functioning; numb" , "Six feet under" , "Gone to meet one's Maker" , "Late; exact" .)