The most common and most likely answer to this crossword clue is the 6 letter word REINED
I've seen this crossword clue in The Times
R
E
I
N
E
D
Buddy explains!

The straight part
We know that "restricted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case restricted = reined.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 REINED mean?
REINED verb- To direct or stop a horse by using reins.
- To restrain; to control; to check.
- To obey directions given with the reins.