The most common and most likely answer to this crossword clue is the 7 plus 4 letter word CHARIOT RACE
I've seen this crossword clue in The Times
C
H
A
R
I
O
T
R
A
C
E
Buddy explains!
The straight part
We know that "A dash of old" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a dash of old = chariot race.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.