Very competitive in the end entering car race (6)
I believe the answer is:
really
'very' is the definition.
(I've seen this before)
'competitive in the end entering car race' is the wordplay.
'in the end' suggests the final letters.
'entering' indicates putting letters inside.
'car race' becomes 'rally' (rally is a kind of car race).
The final letter of 'competitive' is 'e'.
'e' placed inside 'rally' is 'REALLY'.
(Other definitions for really that I've seen before include "Actually, honest to God" , "Do you mean it?" , "Actually, in fact" , "Well I never!" , "In truth - join forces again" .)