The most common and most likely answer to this crossword clue is the 7 letter word MASTIFF
I've seen this crossword clue in Financial Times
M
A
S
T
I
F
F
More clues leading to the result MASTIFF
- Mother has difficult dog
- Corpse covered by Ma Barker?
- Dog is mother’s pet?
- Dog seen in mother’s picture file
- Parent with very difficult dog
- Dog rolling over in the morning, apparently dead?
- Old woman with difficult dog
- Pet is something obtained by student body
- Large dog getting mother tense
- Acorns provided by strong breed of dog?
Buddy explains!

The straight part
We know that "a dog" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a dog = mastiff.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.
What does MASTIFF mean?
MASTIFF noun- One of an old breed of powerful, deep-chested, and smooth-coated dogs, used chiefly as watchdogs and guard dogs.