The most common and most likely answer to this crossword clue is the 8 letter word TRIAXIAL
T
R
I
A
X
I
A
L
Buddy explains!
The straight part
We know that "Part of sponge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of sponge = triaxial.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 TRIAXIAL mean?
TRIAXIAL adjective- Having three axes (e.g having a shape of an elongated oblate spheroid)