The most common and most likely answer to this crossword clue is the 3 plus 3 letter word HOT ROD
I've seen this crossword clue in The Daily Telegraph
H
O
T
R
O
D
More clues leading to the result HOT ROD
Buddy explains!

The straight part
We know that "fast car" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fast car = hot rod.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.