The most common and most likely answer to this crossword clue is the 7 letter word HEAVILY
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More clues leading to the result HEAVILY
Buddy explains!

The straight part
We know that "painfully" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painfully = heavily.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 HEAVILY mean?
HEAVILY adverb- In a heavy manner.
- With a great weight.
- To a considerable degree, to a great extent.
- In a manner designed for heavy duty.
- So as to be thick or heavy.
- In a laboured manner.