The most common and most likely answer to this crossword clue is the 8 letter word PERFECTA
I've seen this crossword clue in Independent
P
E
R
F
E
C
T
A
Buddy explains!
The straight part
We know that "that’s 5 – American, originally (8)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s 5 – american, originally (8) = perfecta.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.
What does PERFECTA mean?
PERFECTA noun- A kind of bet wherein the first and second-place finishers must be predicted in the correct order.