The most common and most likely answer to this crossword clue is the 12 letter word DECOMPRESSED
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The straight part
We know that "relieved" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relieved = decompressed.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
DECOMPRESSED verb- To relieve the pressure or compression on something.
- To bring someone (such as a diver) back to normal atmospheric pressure after being exposed to high pressure.
- To restore (compressed data) to its original form.
- To adjust to normal atmospheric pressure after being exposed to high pressure.
- To relax.