The most common and most likely answer to this crossword clue is the 3 letter word KOI
I've seen this crossword clue in The Times
K
O
I
Buddy explains!
The straight part
We know that "carp" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case carp = koi.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 KOI mean?
KOI noun- Ornamental domesticated varieties of the common carp (Cyprinus carpio), of Japan and eastern Asia with red-gold or white coloring.