The most common and most likely answer to this crossword clue is the 4 plus 4 letter word DEAD WOOD
I've seen this crossword clue in The Guardian
D
E
A
D
W
O
O
D
More clues leading to the result DEAD WOOD
Buddy explains!
The straight part
We know that "It floats/but if it can’t be dumped, it might have to be carried" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it floats/but if it can’t be dumped, it might have to be carried = dead wood.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.