The most common and most likely answer to this crossword clue is the 8 plus 7 letter word THOUSAND GUINEAS
I've seen this crossword clue in The Times
T
H
O
U
S
A
N
D
G
U
I
N
E
A
S
Buddy explains!
The straight part
We know that "Race" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case race = thousand guineas.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.