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

The straight part
We know that "being too sweet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being too sweet = sentimentalism.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.
Crossword Answer definitions
SENTIMENTALISM noun- A liking for sentimental things
- An overly sentimental thing or condition; bathos or sentimentality
- A view according to which morality is somehow grounded in moral sentiments or emotions.