The most common and most likely answer to this crossword clue is the 9 letter word LABELLING
I've seen this crossword clue in The Times
L
A
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Buddy explains!
The straight part
We know that "classifying" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case classifying = labelling.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 LABELLING mean?
LABELLING verb- To put a label (a ticket or sign) on (something).
- (ditransitive) To give a label to (someone or something) in order to categorise that person or thing.
- To replace specific atoms by their isotope in order to track the presence or movement of this isotope through a reaction, metabolic pathway or cell.
- To add a detectable substance, either transiently or permanently, to a biological substance in order to track the presence of the label-substance combination either in situ or in vitro
- A set of labels applied to the various objects in a system.
- The introduction of a traceable chemical group (e.g., containing an isotope or a fluorescent dye) into a protein or other biomolecule of interest so it can be tracked or quantified during experimental analysis.