The most common and most likely answer to this crossword clue is the 10 letter word STEELYARDS
I've seen this crossword clue in Independent
S
T
E
E
L
Y
A
R
D
S
Buddy explains!

The straight part
We know that "They weigh" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they weigh = steelyards.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 STEELYARDS mean?
STEELYARDS noun- A transportable balance with unequal arm lengths.
- A place where steel (and possibly other metals as well) is stored and sold.