The most common and most likely answer to this crossword clue is the 4 letter word CAVY
C
A
V
Y
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The straight part
We know that "Guinea pig" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case guinea pig = cavy.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 CAVY mean?
CAVY noun- A tailless rodent of the family Caviidae and the genus Cavia, with short ears and larger than a hamster; the species Cavia porcellus is often kept as a pet.
- A rodent of any of several species within the family Caviidae.
- A living experimental subject.
- A professional company director, without time or real qualifications for the duties.
- A midshipman in the East India service; (by extension) a low-skilled or non-proficient seaman.