The most common and most likely answer to this crossword clue is the 5 letter word TONNE
I've seen this crossword clue in The Times
T
O
N
N
E
More clues leading to the result TONNE
Buddy explains!

The straight part
We know that "Great weight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case great weight = tonne.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 TONNE mean?
TONNE noun- A metric unit of mass equal to 1000 kilograms. Symbol: t
- A score of 100.