The most common and most likely answer to this crossword clue is the 7 letter word TONNAGE
I've seen this crossword clue in The Guardian
T
O
N
N
A
G
E
More clues leading to the result TONNAGE
- Old news in Times: Queen Elizabeth’s weight?
- Weight putting horse in mood
- No agent misrepresented cargo capacity
- One complaining in distinctive voice seeing capacity of ship
- Weight of silver in 1,000kg
- Tax on wine old news in Times
- Torment breaks character — how much can one bear?
- Pick at sound bags, as weight carried on ship
- Weight carried by horse in match
- Main bulk of sound enveloping The Trout?
Buddy explains!

The straight part
We know that "Main bulk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main bulk = tonnage.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 TONNAGE mean?
TONNAGE noun- The number of tons of water that a floating ship displaces.
- The capacity of a ship's hold etc in units of 100 cubic feet.
- The number of tons of bombs dropped in a particular region over a particular period of time.
- A charge made on each ton of cargo when landed etc.
- The total shipping of a fleet or nation.