Not a single person has energy after midday (2,3)
I believe the answer is:
no one
'not a single person' is the definition.
(I've seen this before)
'energy after midday' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'after' says to put letters next to each other.
'midday' becomes 'noon' (synonyms).
'e' put after 'noon' is 'NO-ONE'.
'has' is the link.
(Other definitions for no one that I've seen before include "who is perfect?" , "Top person" , "top dog" , "Cipher" , "Not a soul" .)