The most common and most likely answer to this crossword clue is the 5 letter word CHAPE
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The straight part
We know that "Part of buckle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of buckle = chape.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 CHAPE mean?
CHAPE noun- The piece by which an object is attached to something, such as the frog of a scabbard or the metal loop at the back of a buckle by which it is fastened to a strap.
- The transverse guard of a sword or dagger.
- The lower metallic cap of a sword's scabbard.