Runner over a long distance has zero energy (4)
I believe the answer is:
nile
'runner' is the definition.
(Nile River is an example)
'over a long distance has zero energy' is the wordplay.
'over' says the letters should be written in reverse.
'a long' becomes 'l'.
'distance' becomes 'in'.
'has' is a charade indicator (letters next to each other).
'zero energy' becomes 'e' (physics symbol as in E=mc2. I am not sure about the 'zero' bit.).
'l'+'in'='lin'
'lin' back-to-front is 'nil'.
'nil'+'e'='NILE'
Can you help me to learn more?
(Other definitions for nile that I've seen before include "World's longest river" , "River at Luxor" , "Cleopatra's river" , "River (which is originally blue and white)" , "River associated with Cleopatra" .)