Runner with zero energy (4)
I believe the answer is:
nile
'runner' is the definition.
(Nile River is an example)
'with zero energy' is the wordplay.
'with' means one lot of letters go next to another.
'zero' becomes 'nil' (synonyms).
'energy' becomes 'e' (physics symbol as in E=mc2).
'nil'+'e' is 'NILE'.
(Other definitions for nile that I've seen before include "River associated with Cleopatra" , "'The longest river, blue and white (4)'" , "Current" , "Long river" , "Cleopatra's river" .)