The most common and most likely answer to this crossword clue is the 4 letter word SOLA
S
O
L
A
Buddy explains!
The straight part
We know that "material used by Lock & Co, et al" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case material used by lock & co, et al = sola.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 SOLA mean?
SOLA noun- A wild plant found in Bengal and Assam, Aeschynomene aspera, having a milky-white, spongy pith used for the manufacture of pith helmets and decorative artefacts.