The most common and most likely answer to this crossword clue is the 12 letter word OUTDISTANCED
I've seen this crossword clue in The Guardian
O
U
T
D
I
S
T
A
N
C
E
D
Buddy explains!
The straight part
We know that "Left far behind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case left far behind = outdistanced.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 OUTDISTANCED mean?
OUTDISTANCED verb- To run further or faster than another, or to finish a race with a large margin.