The most common and most likely answer to this crossword clue is the 15 letter word SYMPATHETICALLY
I've seen this crossword clue in The Guardian
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More clues leading to the result SYMPATHETICALLY
Buddy explains!
The straight part
We know that "thoughtfully" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thoughtfully = sympathetically.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 SYMPATHETICALLY mean?
SYMPATHETICALLY adverb- Owing to or showing evidence of "sympathy", or affinity; happening through or demonstrating correspondences, whether occult or physiological.
- In a manner which demonstrates a sharing in the feelings of others; compassionately.