The most common and most likely answer to this crossword clue is the 5 plus 5 plus 4 letter word UNDER ONE'S BELT
I've seen this crossword clue in The Times
U
N
D
E
R
O
N
E
'
S
B
E
L
T
Buddy explains!
The straight part
We know that "Firmly secured/area where one shouldn't be hit?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firmly secured/area where one shouldn't be hit? = under one's belt.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.