In fact, car race has energy (6)
I believe the answer is:
really
'in fact' is the definition.
(synonyms)
'car race has energy' is the wordplay.
'car race' becomes 'rally' (rally is a kind of car race).
'has' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'rally' enclosing 'e' is 'REALLY'.
(Other definitions for really that I've seen before include "Do you mean it?" , "In genuine way" , "In actual fact" , "Honestly, it's a fact" , "surprised expression" .)