The most common and most likely answer to this crossword clue is the 6 letter word HOPPLE
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The straight part
We know that "Restrain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case restrain = hopple.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 HOPPLE mean?
HOPPLE noun- (chiefly in the plural) A fetter for horses or cattle when turned out to graze.
- To impede by a hopple; to tie the feet of (a horse or a cow) loosely together; to hobble.
- To entangle; to hamper.