The most common and most likely answer to this crossword clue is the 9 plus 4 letter word OVERNIGHT CASE
I've seen this crossword clue in The Times
O
V
E
R
N
I
G
H
T
C
A
S
E
More clues leading to the result OVERNIGHT CASE
Buddy explains!
The straight part
We know that "Not a day patient/bag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not a day patient/bag = overnight case.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.