The most common and most likely answer to this crossword clue is the 5 plus 3 letter word GROSS TON
I've seen this crossword clue in Financial Times
G
R
O
S
S
T
O
N
More clues leading to the result GROSS TON
Buddy explains!
The straight part
We know that "A large amount of weight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a large amount of weight = gross ton.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.