The most common and most likely answer to this crossword clue is the 14 letter word UNCONSCIONABLE
I've seen this crossword clue in Financial Times
U
N
C
O
N
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A
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Buddy explains!

The straight part
We know that "excessive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case excessive = unconscionable.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 UNCONSCIONABLE mean?
UNCONSCIONABLE adjective- Not conscionable; unscrupulous and lacking principles or conscience.
- Excessive, imprudent or unreasonable.