The most common and most likely answer to this crossword clue is the 4 plus 9 letter word DOGS BREAKFAST
I've seen this crossword clue in The Times
D
O
G
S
B
R
E
A
K
F
A
S
T
More clues leading to the result DOGS BREAKFAST
Buddy explains!
The straight part
We know that "Shambles/first meal for Rover?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shambles/first meal for rover? = dogs breakfast.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.