The most common and most likely answer to this crossword clue is the 8 letter word UNPLACED
I've seen this crossword clue in The Daily Telegraph
U
N
P
L
A
C
E
D
Buddy explains!

The straight part
We know that "fourth or worse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fourth or worse = unplaced.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 UNPLACED mean?
UNPLACED adjective- Not assigned a place.
- Not among the first three horses to finish a race.