The most common and most likely answer to this crossword clue is the 10 letter word PACKHORSES
I've seen this crossword clue in The Sunday Telegraph
P
A
C
K
H
O
R
S
E
S
Buddy explains!
The straight part
We know that "They bear heavy burdens" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they bear heavy burdens = packhorses.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 PACKHORSES mean?
PACKHORSES noun- A horse used as a pack animal.