The most common and most likely answer to this crossword clue is the 10 letter word REGRETTING
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The straight part
We know that "Feeling bad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case feeling bad = regretting.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 REGRETTING mean?
REGRETTING verb- To feel sorry about (a thing that has or has not happened), afterthink: to wish that a thing had not happened, that something else had happened instead.
- (more generally) To feel sorry about (any thing).
- To miss; to feel the loss or absence of.
- The act by which something is regretted.