People reverse car with energy (4)
I believe the answer is:
race
'people' is the definition.
(people can mean a national group or race)
'reverse car with energy' is the wordplay.
'reverse' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'car' in reverse letter order is 'rac'.
'rac'+'e'='RACE'
(Other definitions for race that I've seen before include "Run; nation" , "Run fast" , "Competitive speed trial" , "People; dash" , "eg The Oaks" .)