The most common and most likely answer to this crossword clue is the 6 letter word GAGGED
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More clues leading to the result GAGGED
Buddy explains!

The straight part
We know that "under restraint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case under restraint = gagged.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 GAGGED mean?
GAGGED verb- To experience the vomiting reflex.
- To cause to heave with nausea.
- To restrain someone's speech by blocking his or her mouth.
- To pry or hold open by means of a gag.
- To restrain someone's speech without using physical means.
- To choke; to retch.
- To deceive (someone); to con.