The most common and most likely answer to this crossword clue is the 9 letter word SPONGIEST
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "most like foam rubber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most like foam rubber = spongiest.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 SPONGIEST mean?
SPONGIEST adjective- Having the characteristics of a sponge, namely being absorbent, squishy or porous.
- Wet; drenched; soaked and soft, like sponge; rainy.
- Drunk.