The most common and most likely answer to this crossword clue is the 14 letter word SESQUIPEDALIAN
I've seen this crossword clue in The Times
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More clues leading to the result SESQUIPEDALIAN
Buddy explains!

The straight part
We know that "Very long" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very long = sesquipedalian.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 SESQUIPEDALIAN mean?
SESQUIPEDALIAN noun- A long word.
- A person who uses long words.
- (of a word or words) Long; polysyllabic.
- Pertaining to or given to the use of overly long words.