The most common and most likely answer to this crossword clue is the 7 letter word DRINKER
I've seen this crossword clue in The Guardian
D
R
I
N
K
E
R
More clues leading to the result DRINKER
Buddy explains!

The straight part
We know that "Sponge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sponge = drinker.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 DRINKER mean?
DRINKER noun- Agent noun of drink; someone or something that drinks.
- Someone who drinks alcoholic beverages on a regular basis.
- A device from which animals can drink.
- A pub.