The most common and most likely answer to this crossword clue is the 8 letter word SPONGING
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "as parasites" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as parasites = sponging.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 SPONGING mean?
SPONGING verb- To take advantage of the kindness of others.
- To get by imposition; to scrounge.
- To deprive (somebody) of something by imposition.
- To clean, soak up, or dab with a sponge.
- To suck in, or imbibe, like a sponge.
- To wipe out with a sponge, as letters or writing; to efface; to destroy all trace of.
- To be converted, as dough, into a light, spongy mass by the agency of yeast or leaven.
- The application of a sponge.