The most common and most likely answer to this crossword clue is the 6 plus 9 letter word GOLDEN RETRIEVER
I've seen this crossword clue in Financial Times
G
O
L
D
E
N
R
E
T
R
I
E
V
E
R
More clues leading to the result GOLDEN RETRIEVER
- Jason the bird collector?
- Kind group almost always billets old dog
- Dog, familiar in type, leapt finally with energy into stream
- Good and mature terrier, even, disturbed other breed
- Vet, Leo, with derringer, shot dog
- Gentle ride Rover represented for man’s best friend?
- Dog will deliver, no regret otherwise
- One in mourning embraces aged, retired man’s best friend
- Dog barking longer, tired always
Buddy explains!
The straight part
We know that "man’s best friend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case man’s best friend = golden retriever.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.