The most common and most likely answer to this crossword clue is the 2 plus 7 letter word ST BERNARD
I've seen this crossword clue in The Times
S
T
B
E
R
N
A
R
D
More clues leading to the result ST BERNARD
Buddy explains!
The straight part
We know that "Dog" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dog = st bernard.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.