The most common and most likely answer to this crossword clue is the 5 plus 5 letter word UNDER WRAPS
I've seen this crossword clue in The Times
U
N
D
E
R
W
R
A
P
S
More clues leading to the result UNDER WRAPS
Buddy explains!

The straight part
We know that "Hidden" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hidden = under wraps.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.