The most common and most likely answer to this crossword clue is the 8 letter word NEOPRENE
I've seen this crossword clue in The Times
N
E
O
P
R
E
N
E
More clues leading to the result NEOPRENE
Buddy explains!

The straight part
We know that "Rubber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rubber = neoprene.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 NEOPRENE mean?
NEOPRENE noun- A synthetic rubber, a polymer of chloroprene, commonly used in wetsuits, laptop sleeves, orthopedic braces, electrical insulation, liquid and sheet-applied elastomeric membranes and flashings, car fan belts, etc.