The most common and most likely answer to this crossword clue is the 9 letter word SECTIONAL
I've seen this crossword clue in The Sunday Telegraph
S
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C
T
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Buddy explains!

The straight part
We know that "Relating to parts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relating to parts = sectional.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 SECTIONAL mean?
SECTIONAL noun- An item of furniture composed of modular sections; usually specifically a sectional sofa
- A tournament or match held at the section level, typically between the regionals and the championships
- A band sectional, in which one section of a band or orchestra practices separately
- Relating to a section.
- Separating into sections.
- Relating to conflict between areas.