The most common and most likely answer to this crossword clue is the 3 plus 2 plus 4 plus 5 letter word SIT ON ONES HANDS
I've seen this crossword clue in The Times
S
I
T
O
N
O
N
E
S
H
A
N
D
S
Buddy explains!
The straight part
We know that "to stay idle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to stay idle = sit on ones hands.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.