The most common and most likely answer to this crossword clue is the 9 letter word STRAPPING
I've seen this crossword clue in The Daily Telegraph
S
T
R
A
P
P
I
N
G
Buddy explains!

The straight part
We know that "Massive/binding material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case massive/binding material = strapping.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 STRAPPING mean?
STRAPPING verb- To beat or chastise with a strap; to whip, to lash.
- To fasten or bind with a strap.
- To sharpen by rubbing on a strap, or strop
- Adhesive plaster for strapping injuries.
- A length of narrow material to be used for straps, or straps collectively.
- A beating with a strap.
- The act of fastening with a strap.
- Having a sturdy muscular physique; robust.
- Of a young woman: full of vigor; lusty.