Nobody with energy after twelve (2,3)
I believe the answer is:
no one
'nobody' is the definition.
(not a single person)
'energy after twelve' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'after' means one lot of letters go next to another.
'twelve' becomes 'noon' (twelve noon).
'e' put after 'noon' is 'NO-ONE'.
'with' acts as a link.
(Other definitions for no one that I've seen before include "Top person" , "top dog" , "Failure" , "Not anybody" , "the street's first house?" .)