The most common and most likely answer to this crossword clue is the 9 letter word RETRIEVER
I've seen this crossword clue in Financial Times
R
E
T
R
I
E
V
E
R
More clues leading to the result RETRIEVER
- Remember a small bit of Ronnie Barker
- Steep watercourse engulfs European dog
- Golden go-getter?
- Relation of terrier vet treated, wasting time
- Irreverent drunk docking tail of deacon’s dog
- Hound person to set the situation right?
- Screwed terrier, biting very English, fetching type
- Barking terrier, extremely virile dog
- Dog expert vows initially to stop terrier barking
- He brings back the dog
Buddy explains!

The straight part
We know that "Relation of terrier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relation of terrier = 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.
What does RETRIEVER mean?
RETRIEVER noun- One who retrieves something.
- A type of gun dog that retrieves game for a hunter.
- A tool for retrieving materials from the body after an operation.