The most common and most likely answer to this crossword clue is the 9 letter word SORROWFUL
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More clues leading to the result SORROWFUL
Buddy explains!

The straight part
We know that "Sad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sad = sorrowful.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 SORROWFUL mean?
SORROWFUL adjective- (of a person) exhibiting sorrow; dejected; distraught.
- Producing sorrow; causing grief.