The most common and most likely answer to this crossword clue is the 5 plus 3 plus 4 letter word CROSS THE LINE
I've seen this crossword clue in Financial Times
C
R
O
S
S
T
H
E
L
I
N
E
Buddy explains!
The straight part
We know that "Complete the race/do it unacceptably badly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complete the race/do it unacceptably badly = cross the line.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.