The most common and most likely answer to this crossword clue is the 13 letter word TERMINOLOGIES
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Buddy explains!

The straight part
We know that "Nomenclatures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nomenclatures = terminologies.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 TERMINOLOGIES mean?
TERMINOLOGIES noun- The doctrine of terms; a theory of terms or appellations; a treatise on terms, a system of specialized terms.
- The set of terms actually used in any business, art, science, or the like; nomenclature; technical terms.