The most common and most likely answer to this crossword clue is the 10 letter word FREELOADER
I've seen this crossword clue in The Guardian
F
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Buddy explains!

The straight part
We know that "Sponger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sponger = freeloader.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 FREELOADER mean?
FREELOADER noun- One who does not contribute or pay appropriately; one who gets a free ride, etc. without paying a fair share.
- An individual who takes expired unsold merchandise from the back of supermarket premises.