The most common and most likely answer to this crossword clue is the 3 letter word OBO
O
B
O
Buddy explains!
The straight part
We know that "Mixed-load carrier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mixed-load carrier = obo.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 OBO mean?
OBO noun- A shamanistic cairn found in Mongolia, usually made from rocks or wood and found on or around mountains.