The most common and most likely answer to this crossword clue is the 11 letter word HEAVYWEIGHT
I've seen this crossword clue in The Times
H
E
A
V
Y
W
E
I
G
H
T
Buddy explains!
The straight part
We know that "of substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of substance = heavyweight.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 HEAVYWEIGHT mean?
HEAVYWEIGHT noun- A very large, heavy, or impressive person.
- The professional boxing weight class for boxers weighing more than 190 pounds; a boxer in that division.
- (uncountable) A similar division and contestant in other sports.
- Of the heavyweight boxing (or similar) division.
- Being relatively heavy.
- Being a leader in one's field.
- Important or impressive.