The most common and most likely answer to this crossword clue is the 16 letter word DOUBLE-BARRELLED
I've seen this crossword clue in Independent
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More clues leading to the result DOUBLE-BARRELLED
Buddy explains!
The straight part
We know that "of two parts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of two parts = double-barrelled.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 DOUBLE-BARRELLED mean?
DOUBLE-BARRELLED adjective- Twofold, having a double purpose or nature.
- (of names) Having two separate parts, often adjoined by a line (or sometimes a space), e.g. Wright-Phillips.
- Forceful, powerful (like a double-barrelled shotgun).